One reason might be that those who were incumbents were already well known, so harder to demonize.
Biden was very hurt by the attacks on his age and competency. His team made a gigantic error not asking to reschedule the debate when he was sick. The terrible debate performance amplified those concerns, while the Trump lies and even strange comments were seen as "just Trump".
Harris was not given the high profile that Biden and Gore were given as VP, even as she did participate in everything. Consider Obama put Biden in charge on some economic issues. Harris was not given the lead position on infrastructure. Instead, she got the border which was NEVER going to seen as successful even if it became measurably better than under Trump.
In retrospect, the Republicans took the joyful, diverse campaign and distorted it by othering most of the prominent people, characterizing Harris, a very articulate, thoughtful person as having a strange laugh and not making sense ( chutzpah when their hero spoke of Hannibal Lectur etc )
They also succeeded in convincing many people that the Trump years were economically better for them. This was objectively not true and by 2021 Trump policies including the unsustainable tax cuts had created an economy in grave danger of going into recession. Biden and his team avoided that and turned things around in a better way than any economic expert thought possible. Because of the latter, the question of whether you were better 4 years ago was not even a fair question. However, most people could not even fathom an answer to the question of whether you would be better now had Trump won.
Throw in the absolutely horrible ads created to demonize Harris on things like the border, where the US has long had many xenophobic people, and transphobia, where Harris comments were cherry picked and you can see they negatively defined Harris, while cleaning up Trump. Living in VT, I saw few of these ads until my husband watched the world series. All this in addition to racism and misogyny.
There were mountains of ads against the Senators too, but they were less able to distort who these known people were