Regardless of what Biden has done for the average American, his approval rating has been really bad for a while now. It may not be right or fair, but that's what it is.

Despite Harris not actually being the incumbent, the Republicans managed to associate her with Biden and she more or less embraced it. Then the Republicans were able to frame themselves as the challenger to an unpopular incumbent president and it's not surprising they did well.

Of course, it didn't help either that Harris is the VP either. Perhaps if we had a primary and managed to pick another candidate that could distance themselves a bit better from Biden things would be different.