Why do contemporary 'Midcentury' spaces so rarely relate to the original period interiors they are inspired by?  

I will guess what I think you mean after looking at this cafe as it has crossed my mind with most eras.
http://fryupsgoodornot.blogspot.com.au/2011/09/regency-cafe-pimlico-lond...
Most eras like this 50s cafe is mostly made up of the common persons interior.This cafe inside was likely a common theme of laminex and common 50s fittings.Today we look back and think it quaint or cool kitsch.
In effect the low end run of the mill common stuff was what 99% of the population actually grew up with and not the cool image we imagine when looking back at an era.
People these days when looking back at an era generally view only what is shown in architectural or home magazines of the time.These usually show the best interiors of the era viewing through the lens.That of course is for the demographic the magazine is aiming at.Not always but often the better the magazine the better the interior and more high end the furniture designs.
I imagine if kids today get the design bug when they are older they might be looking to collect the high end designer pieces of today seen in current magazines.Maybe they will have fond memories of growing up with cheap Chinese furniture and junk,knock offs and Ikea furniture that falls apart after 2 years.They will dig up the old ipad from their grandparents garage and think wow how funny,remember this old thing when you actually had to touch it instead of using holograms and telekinesis like today.So retro.