Biden beat Trump during COVID and when Trump was the incumbent. Being, "not the other guy" works a lot better when you aren't currently the president when the bad things are happening.

Given high inflation that was never balanced by wages or deflation and an ongoing sponsored genocide, "more of the same" and a near complete lack of major policy positions means hewing close to the unpopular incumbent.

Also she literally never won a primary. She lost to Yang lmao. This was donors shoving a pro-gemocide empty suit candidate down peoples' throats. "Winning" is a secondary goal to choosing the candidates.