I'm a bit surprised by this. I assumed more people would tune in to Trump to see what crazy / off the wall things he would say, and Biden just kind of is who he is, no expectations of any drama there. So maybe this is an indication that people are just tired of Trump, just done with it.
For myself, I left the original debate a lot more sickened and disheartened than I expected to. Even though I pay more attention to politics than the average person, I had no desire to watch either Trump or Biden last night - Trump because I just can't stand hearing him talk any more, and Biden because I already know I'm voting for him, I don't need any more convincing. I think a lot of people are just tired of it all, and are ready to move on.