One of the primary reasons for this is RESENTMENT - the same resentment that has led to an increased number of those supporting socialism in America.
A vibrant middle class requires the freedom for people to improve their socioeconomic level and ‘climb the ladder’ via their own efforts and drive. A significant swath of the populace resents this - as it leads to people ‘just like them’ achieving more than them and having more financial resources. It's jealousy, and resentment - the same kind that fueled the Bolsheviks, the Chinese cultural revolution, and those who handed the reigns of power and the resultant destruction of Venezuela to Chavez and then Madura. These are the same types of people (with the same resentments) that AOC appeals to.
These same people don't mind movie stars, pop musicians, athletes, etc. having lots of money - because becoming wealthy in those ways is somewhat like winning the lottery. Few from within their ranks ever achieve success in those ways. If they do, it is attributed to their good fortune (they were lucky to be born with that talent or that athleticism). It doesn't reflect back on the crowd, as an indictment of the efforts they didn't make.
The general ways that people have traditionally climbed the ladder in America don't usually lead to them becoming the ultra-rich. Becoming a practicing physician or lawyer does make most ultra-rich. Starting a small business and growing it generally doesn't lead to people becoming ultra-rich. Working two-jobs for years and wisely investing your money can lead to a rise of several rungs on the ladder - but generally doesn't make one ultra-rich. That said, all of these things can lead to a substantial improvement in the lives of these people and their families. These are the people the AOC crowd resent - people from within their own ranks who have worked hard to better themselves. These are also the people who have traditionally made up the ‘vibrant middle class’ you refer to.
Affirmative action, unions dictating special benefits for themselves, government handouts, cheap foreign labor (legal and illegal), a system that encouraged forever debt are just some of the contributing factors.