People did not bet a lot on Biden, in fact Trump was the heavy favorite from what I remember when live betting was going on and even on the last day where only a few electoral votes/seat were left the odds were still up and I was surprised if that was a mistake because Biden win was almost certain and odds on last day though were low around 1.1 but still juicy considering he already almost won it. But Trump was favorite earlier until counting began.
I think people didn't bet that much on Biden because his odds where so strong in his favor. The payout was fairly small compared to Trump. Initially I bet on Trump because there was a decent profit to be made if he actually won. But then the polls kept getting more and more in favor for Biden so I ended up actually closing out my bet. I just didn't see any chance for Trump to win with so many things going bad for him. Especially the corona pandemic and how it was handled cost him the election in my opinion.