Massive theological/philosophical shifts. Architecture is a good berometer of what a peoples believes to be true of the world. 
Post 1920s architecture became increasingly statist, anti-theistic, and anti-human. 

Post-industrialism also saw an increasing loss of craftsmanship and skills over the course of the past 200 years. We simply don't know HOW to make buildings like this anymore.  

Add a century of fiat and high time preference to that backdrop.

There is a slow but present resurgence of excellent architecture, though. New Urbanism and New Traditional Architecture are two movements happening in Europe and America right now. Not the caliber of architecture you picture here, but a welcomed development nonetheless. 

Check out newtrad.org
Here is an example of New Urbanism woodburymoscow.com 

 Hmm... Good analysis. The men in gray have convinced us to forget art to save time.

I've been thinking about this for years now but had no idea about any of this: newtrad.org! Yay!
This brightened my day. 

 Thanks