My pet theory is that the establishment wing of the party (that largely controls the DNC) wants to have a specific coalition that keeps them in power within the party. Like, if Democrats wanted to be the party of the working class or appeal to rural voters, they could but would require leadership that isn’t from New York, San Francisco, or other similarly rich places.

I think it’s simultaneously less conspiratorial and more nefarious than that: the establishment wing of the party likes power and power means control over campaign funding, so they pander to large corporate donors by suppressing anti-corporate populists.