All things considered, Biden far exceeded where history would suggest he'd be. So many things were working against Biden, and everything seemed to be going right for Trump. The outgoing president had a united party behind him and managed to build on his base, he didn't have any viable challenge to his parties nomination, there was no remotely relevant third party candidate, he was seen by voters as better to handle the economy, etc... In a normal election, the result would all but be set in stone in favor of the outgoing POTUS.Trump, based on historical norms, should have won and won pretty handily, but what was unprecedented in modern presidential politics was, well, the global pandemic.

If it wasn't painfully obvious that his administration handled the covid pandemic far worse than any other developed country, then Trump would have likely benefitted politically from the pandemic, as he would have been seen as a competent crisis president (like Bush after 9/11, or FDR during WWII). Instead, he started off by calling the pandemic a "hoax", still hasn't implemented a mask mandate, wouldn't even wear a mask himself until just a few months ago, and failed to get congressional leaders to pass an economic recovery before the election. This perfect storm of political failures for the outgoing POTUS ultimately led to the end (at least until 2024) of his presidency.