Similar to here there's obviously a big element of voting against the party/candidate you dislike as much as for the one you do like.
Others have already pointed out that some will have voted against Harris based on her race and gender. But there must also be a bit of blame apportioned to the DNC. The arrogance and hubris to think they could just run Biden without voters noticing his mental decline, and then to switch him out as late as they did, bypassing their usual democratic processes for selecting a candidate in primaries must have lost a number of voters before campaigning even began.
Others have already pointed out that some will have voted against Harris based on her race and gender. But there must also be a bit of blame apportioned to the DNC. The arrogance and hubris to think they could just run Biden without voters noticing his mental decline, and then to switch him out as late as they did, bypassing their usual democratic processes for selecting a candidate in primaries must have lost a number of voters before campaigning even began.