I agree that aside from Biden and Trump, there are no candidates anymore, those are the two that will go ahead and win it, and there is really nothing wrong with that, I agree that those two will be fighting it out and one will win, so rest of the candidates makes no sense, but really? Michelle over Kamala? That is silly.
It must be because the attention Michelle Obama has received in this presidencial campaign compared to the attention Kamala Harris has. If you pay some attention to the media on the United States, it would not take much time for you to realize Kamala Harris is the kind of politician who is easier to ignore than the average person in Washington. The media give even more attention to Nancy Pelosi, and she is not even the speaker of the house of representatives anymore, you know. The right wing media used to attack her, but she has become to irrelevant that, in the event Biden shows himself to be unable to continue with the campaign, the democrat party would never opt to push Harris into the White House, they would lose.
In that context, Michelle Obama is better recalled as the wife of the President Obama, and has showed to have more charisma when facing the general electorate, that is why she is preferred by bettors over Harris. They know the Democrat party are more likely to favor Obama than her, even though Harris is already into politics and being the second on command after the president.
It would be safe to say Harris is sort like the DeSantis of the Democrat party. You know she exists, but nobody pays much attention to her anyways. A very low profile.