I think to win the election it's the middle voters you have to win over. Hardcore Trump fans are never going to vote Democrats, so it's the floating voters or the ones who have some doubts that you need to sway.
Trump unfortunately appeals to a certain type of voter. Not exclusively, but certainly many of them. He's says outrageous things that no other politician would touch - very nationalistic things that a certain demographic laps up, including some of those in the middle.
Harris isn't just black but she's also female, and has a reputation for being very left-leaning. It's an enormous leap for those middle voters to go from gungho, racist Trump to a left-wing black woman. Maybe a white woman, or a black man - but bundle them both together and throw in her left-wing reputation and she has a mountain to climb.
I also don't think she's been particularly visible in the last four years, and doesn't seem to have built up a strong reputation as an exceptional politician. It's said that she's weak on policy and hasn't really made any progress on the areas she was responsible for.
On a personal level, I don't get much of a presidential vibe from her. I'm not especially impressed when I see her, much as I'd like to be.
It's a weird one though because Michelle Obama polls very well, even though she allegedly won't consider running. Although she's black and a woman, Michelle seems to have such popularity that she's able to overcome the twin "handicaps" of being black and female! Possibly that's partly due to the fact that Obama is still held in high esteem by many, and his reputation will help. But I also think it's the way she's handled herself - she's always appeared to be a strong, eloquent woman who holds traditional family values, and puts her family at the centre of all she does. That will appeal to many in the middle - I think her qualities appeal to both sides, transcending the usual tribal divisions. She appears confident and assured, and nothing ever seems to faze her. The conservative voters in the middle will appreciate her values and how she's always conducted herself (no shady past like certain other First Lady candidates...). Hardcore Trump voters won't go for her, but they're not going to vote for any Dem so it's irrelevant.
I think sadly Kamala Harris is a complete non-starter - I think she's probably got even less chance of winning than Joe. But if Joe did stand down, would she expect to slot into his place? It's a real slap in the face for her to bring in another candidate above her.