Biden's policies, all of which Kamala would continue unchanged, are extremely unpopular. She would continue gifting bombs to Israel to commit genocide with, do nothing about housing, medical care, reproductive or trans rights, policing...

Giving people time to realize she's just going to be more of the same would probably hurt more than help. Especially in regards to Palestine. Liberals are still leaving in dream reality where she really does super care a lot, really, she just thinks you need to beg in exactly the correct way to not slaughter children.

My thought is that it's more the opposite why they haven't put Kamala in the Oval Office yet. Dems/Kamala want to run away from Biden while maintaining all his policies. They just need a new candidate that isn't tainted by Biden and the current policies while not addressing those policies exist at all. So if Kamala had the power, then she would have to answer for wielding it. The current arrangement allows her to be a perfect figuredhead for 'something else' aka 'not Biden or Trump'. Any criticisms of current policies can easily be blamed on Biden. She gets to run against both of them in a weird way.