You're engaging in class reductionism. The main reason is of course her socialist identity. But, to ignore that she is a black woman from the south would be like ignoring the attacks on Abdul for being Muslim (from both parties).

A black socialist woman is significantly more dangerous to the Democratic party as their core voting base in the south is comprised of lifelong democratic black voters that they are trying to distance from DSA. "DSA is all privileged white people" you hear both parties regurgitate. They clearly see her as threat that will push a large part of their base further left.

It's ok to analyze incentives through intersectionality. The Democratic party establishment is. They are just using it to keep power. Don't ignore it because the main reason they are against her is because of her socialist policies. That doesn't mean there isn't multi things that are a threat from her, because of her identity, that introduce contradictions to their existing narratives.

Edit: This is not to say that black voters aren't extremely more left than the average American on average, they are. But, a large portion of the base of the democratic party, those that vote in primaries, are more likely to vote for the parties pick than an insurgent DSA candidate. A black woman from DSA contradictions their entire narrative. Ignoring that is ignoring how they are attempting to use identity politics to attack socialism.