Because Kamala didn't come across as the stammering idiot that they see in edited youtube videos. That's pretty much it.
Trump hammered her on immigration and the economy, and didn't take the bait, which is what everyone said he needed to do. So yeah, I don't get it. His campaign is getting traction, he's leading (by a lot more than you might think), and he just needs to get back out on the campaign trail, which is REALLY where he makes hay anyway. And bring it home. If Kamala had been winning, there is no way they even DO this debate. Her even agreeing to this was like pulling your goalie in the last few mintues of a game when you're down. No, Trump didn't score an empty netter, but he still won.