Because Trump lies. It’s old news. There was no way that debate was going to play well for Biden. He had nothing to gain. People voting for Trump are voting for Trump, people not voting for Trump are either Voting for Biden or staying home. Biden has to convince people not to stay home, not convince people not to vote for Trump.
The people that vote for Trump are people so sick of the current state of politics they want to burn it all down. Very few actually think he’s a great president. He’s a troll vote. Biden’s performance made more people want to watch the world burn.