Because they thought they could continue hiding Joe's cognitive decline from the electorate. They'd been doing it with relative success for quite some time. It wasn't until that disastrous debate that the elephant in the room was just too big to ignore. What they should have done, in my opinion, was 25th Joe (I'm Canadian, so forgive me if I have the amendment wrong). If he wasn't fit to run, he wasn't fit to lead. Let Kamala be president and run from an incumbency position because there is something called an incumbency advantage. Then she should have taken steps as president to differentiate herself from Biden and then put whatever they had to in place to LISTEN to the concerns of the people. But of course that's a ll common sense and we know the Dems don't have any of that.