I think this is a really interesting question, and it's one that I've pondered quite a few times.
I've thought for a while that Kamala Harris might struggle to win an election. Being brown and female absolutely would be a problem for some voters, plus she's perceived as being more left wing than some others. Add to this the fact that she's not shone on key areas like immigration which Biden very kindly handed to her to deal with, and it's not an appealing combination for a certain demographic.
As PP said, it's not about winning over Dem voters because let's face it, no one with that political persuasion is voting for Trump. It's the floating voters, the ones in the middle that she needs to capture. And even better, she needs to capture some of the more moderate, centralist Republicans who aren't best pleased about voting Trump but at the moment still plan on doing so...
Those voters that need to be converted aren't going to be impressed with a brown-skinned female who leans further to the left wing, and in addition has done a poor job on immigration - which many cite as a key issue.
I wonder if part of the reason that Michelle Obama polls so well is that she appeals to conservative family values? I can remember lots of talk about their family dinners. She's got quite a wholesome reputation, none of the sleaze that came with the Clinton name. My understanding is that she's also not a left wing as Kamala. So maybe all of this appeals to centrists, even if they might otherwise have considered voting Republican? Lots of Dems will love Michelle (for many, many reasons) - but her ability to attract floating voters or moderate Republicans is what's interesting to me. I think her emphasis on family and core moral values might be one of the reasons those further to the right might have been willing to consider her.
Obvs we all know that Michelle won't stand but it's interesting to try and unpick why she polls so well because it could help to identify someone who could beat the orange man-child.
I feel sorry for Kamala because with a different Republican candidate, she might have stood a chance. Especially if she had some time to build up a positive public reputation with some better projects. She's clearly a talented woman with potential - I just think maybe now isn't her time. However, I would absolutely LOVE to be wrong.