Because most of them are outspoken women or the double whammy, women of color.
Sanders has the old white guy advantage but being an Independent will forever lock him out of the top tiers of the Democratic party.
To be fair, a large establishment puts out tons of propaganda to segment the population enough with a bunch of infighting and focus on unimportant details that by design keeps them from successfully coalescing around an individual who has a realistic chance of upsetting corporate+state power.
A lot of people seem to be under the impression that politicians, as they are now, actually run things. They are simply stewards to the people who run society: corporations and various institutions of the state like the state department and CIA whose whole being is seeking to control society and other countries to serve American corporate interests. Biden and most of the politicians currently are funded and kept in power by these private entities.
There is a reason Joe Manchin went and talked in-depth and gave thought-out answers to dark money figures on that call that was leaked instead of to his voters and the public at large. He knows who butters his bread and so do all these other politicians. So they are always going to act in the interest of privatization and corporate power.
When someone comes along who has a serious chance of upsetting things, then the media, various politicians, and various actors usually getting their paycheck from corporate entities pull out all the stops to smear and stop that person from being successful.