I don't know. I feel like there was a segment that had it in their heads that 81 years old is too old to be running for president. Even though Trump is just 3 years younger than him. Also the media was locked in on attacking any flub Biden made, while giving Trump a pass.
The media and voters was likely going to make tougher re-election fight, than it ought to be.
"Never confuse a single defeat with a final defeat." - F. Scott Fitzgerald