why didn't modern architecture/design save north america?  

The problem with this question is...
I can't think of any continent that has ever been saved by any architectural ideology. I just don't think "saving" continents is the right way to ask your question.
Still, your question can be redesigned into a very important compound question.
Why did Modernism achiever wider acceptance and why wasn't it able to sustain itself?
Koen gave one very illuminating answer to both of these questions previously.
Modernism rationalized the practical functions of architecture and design, but disregarded rationalizing the symbolic functions. This lead first to it not achieving broad acceptance, and second to not being sustainable as a ideological movement.
But I would like to supplement what Koen said a bit by adding a model of context that the modernist ideology was operating in when it failed to gain broad and sustainable acceptance. Let us remember that modernism took place within the context of an urban land economy that might be more usefully be called an urban ecology.
In a robust, diversified forest, or bee hive, or other ecological community, one would not expect a single species in the forest, or single type of individual (like a drone) or a single uniform form to suddenly come to completely dominate an ecological community. Ecologies are reconciliations of complex contextual constraints and interplays.
Frankly, if one thinks of a city as an ecological community, modernism achieved very broad acceptance in its time, at least in commercial buildings; less so in housing, but its acceptance there was still very significant. Buildings can last along time and they are woven into a complex of physical, political, legal, social and economic infrastructures that support and perpetuate them. Hence, there is incredible inertia in urban ecologies. They are dominated by legacy systems.
Further, the demand for more space is always, even in the biggest growth periods, a small percentage of the total building stock existent. So: even if modernism had achieved total acceptance by the society, and had been able to sustain that acceptance, it would have likely taken over a century or two of building cycles to completely eliminate the non modernist buildings from the landscape.
Finally, urban ecologies are places where architectural ideologies compete for the building action. When modernism failed to address the symbolic functions of buildings for people, modernism not only failed to satisfy people on a certain level but left open a path for a competing ideology to satisfy that human desire for symbolic function. Post modernism did that albeit inadequately.
So: if one combines the shortcomings of Modernism, which limited its appeal and its sustainability, as described by Koen and then you add in the contextual constraints on land use succession in a complex urban ecology, plus the competition from Post Modernism, I think it becomes clear why Modernism unfolded and receded as it did.

well said!
thank you for your careful analysis of the poorly written question. it makes complete sense. these modern architectural symbols, never really attached themselves to the public. your explanation proves why modernist residential architecture failed. disregarded symbols have no life. despite the odds joe eichler had some success, but probably more based on his homes great value to the consumer as opposed to a new honest philosophy in residential architecture.

The question is not only ...
...interesting, it is also a complex one. dcwilson certainly adds a number of aspects that have been overlooked in the past. I do not believe that architects and designers "...all naievely thought.." Ever since the first modernist manifesto, the need to "educate the masses" has been central to the ideology and has been expressed time and again. Many "design centers" were build with the specific goal to educate people. In other words, they all knew how reluctant the users and consumers would be. As dcwilson points out, the technological innovations have been widely adopted. Steel and concrete frame constructions, curtain walls etc. have invaded the city centers. (ironically it was the curtain wall technology that allows these buildings to be so easily "re-cast" in a post-modern version).
In product design, many technological innovations have been used first within "modernist" applications, but there again, only the technology survived. It seems historically wrong to pretend that there was this kind of optimistic expectation. Most of the leading architecs were strong authoritarian figures.Van De Velde, Gropius, Le Corbusier were unflexible personalities, convinced beyond reason of their own righteousness. Mies v.d. Rohe, Jacobsen, Aalto had a somewhat warmer personality, but they to, were not very receptive for public reaction. To expect a positive response from the population without involving them in the process can not be called naive. By the nature of their products designers have always been closer to the users because a product does not exist unless it gets sufficient public support, but even in that field designers had...and some still have this "...I know better..." attitude.
Both architects and designers still have great difficulty accepting that they exercise their profession in "delegated" role. The fundamental right to shape ones environment belongs to all of us. Architects like Lucien Kroll are still very exceptional in the architectural landscape. Generally spoken, early modernism in architecture did not show much respect for the masses either. Look again at Le Corbusier's "plan voisin" for Paris...who would want to live there?. Strangely enough post-modernism made the same elitarist mistake. When Paul Goldberger insisted on the importance of historical references and on the specificity of "place" in post modern architecture, Charles Jencks protested and insisted that these criteria were too marginal. Post-modernism was a according to Jencks a language. Unfortunately the language was again the language of an elite.

What Koen says about the intentions of the famed modernists...PT1
is crucial. He is right. These were not naive men. The vigor, rigor and focus that they applied in pursuing their philosophies of modernism makes that much clear. No one steps into the hard ball of commercial real estate development in any big city in ANY capacity and survives, much less flourishes, without being VERY saavy and learning very fast. These men knew you had to drive very hard bargains AND be strategically and operationally shrewd to realize their visions in even one large building. The two hardest positions in real estate development from which to advance an agenda about what will actually be built, if a strong developer is involved, are architect and land owner. The architect puts up time and lots of it (i.e., he becomes substantially invested by sunk costs) to generate a feasible space program with distinctive and appealing aesthetics. His fee is a percentage of total project budget and he has no way of knowing what that number in fact is, because he doesn't count it up ultimately; the developer does. He also can't really insure away the risk of his position the way several other players at the table can. And if he proves too uncooperative with the developer and too inept at politics with the construction lender, and the city planners, they converge on his design from all directons and disfigure it into something that will cut costs, not challenge the conventions of design already in existence, increase margins, and so stain the architect's reputation with an indifferent, or outright ugly building for the rest of his life. Why can they do this to the architect? Because they all get paid no matter how awful the building is AND they get taken out by the long term lender and equity vehicles that sucker investors if necessary to leave the developer and construction lender whole. In short, none of them are judged on how good or bad the building is, but rather on how much money it makes. An architect on the other hand, needs for the building to be profitable AND distinctively good looking to stimulate more business. True the architect can sue for his fees, but that won't make the building pretty, will it? And some architects can abuse their role one or two times, before being blackballed, crushed, or both in a subsequent project.

What Koen says about the intentions of the famed modernists...PT2
Frankly, the great modernist architects had to devise complicated, effective strategies to ensure their great buildings were built as they envisioned, and they had to be master politicians, or their buildings would have been ground up into mediocrities like so many other buildings have been. IMHO, there are no great developers or key institutional lenders that can resist wrecking a great design. They stand to make too much money cutting costs and corners when they plug out the construction loan at a 120 percent of actual project cost. Only a shrewd architect can by hook and/or crook impose the design envisioned.
To digress a bit, the land contribution position is also a very difficult one to press a building agenda from because its only 20 percent of the value of the project. Hence,one typically has little contractual say about what happens, beyond a ground lease specifying terms of consideration. And once contributed, saavy developers and institutional lenders pretty much have their way with the site. But enough of that.
The point is: Mies couldn't just walk in and say here's the Bauhaus descended design, build it. He had to bullshit the bullshitters, so to speak. He had to make people believe who's business it was not only to doubt, but to scam right up to the limit of the law (sometimes well beyond it) and grab control and dictate building outcomes against all challengers. Trust me, these developers and construction lenders were NOT dazzled by the ideology of modernism. They were persuaded by Mies' ability to promise them cost advantages, marketing advantages due to uniqueness of silhouette on the skyline, space programs that could attract tenants looking for flexible space, or prestige space and so on. And he had to be charismatic enough that the developers and construction lenders could go to their downtown financial clubs and brag that they were bankrolling a structure by a celebrity architect that would become the most talked about building in their city for a time. That's how it works IMHO; that and reputedly: some cronyism, some envelopes to aldermen down town (depending on the town), and understandings with contractors and especially the steel, sand, gravel, glass and concrete sub contractors that a certain percentage of the materials may wind up missing.
No, these modernist architects were not naifs, at least for long.