1) taxes that are too high
2) regulations that are too numerous and too costly
3) too much government borrowing and gov. debt
4) too much inflationary increase in the money supply
5) too much government spending and deficit spending
6) anti business environment with anti rich, anti profit, anti property rights attitudes
7) out of control pro immigration policies
8) lack of knowledge of the boom bust cycle and what causes it and how to properly get out of it
9) redistributionism
10) welfare statism and social justice
11) egalitarianism
12) increased socialism
13) labor unionism
14) increased government interventionism and force
Put all these together and they will overcome technological progress, and the result will be economic stagnation or retrogression
Since 1971 the upper two bands have expanded by 7% - these folks didn't appear out of nowhere, they were the middle class moving up, which normally would be called a good thing.
Looking at the big picture, the 11% loss of the middle class is composed of 7% “moving up” and only 4% “losing out” and falling in relative standard.
Except for the social justice and income redistribution crowds this could be seen as an overall success for the system.