Pelosi, Schumer, and Jeffries have been coordinating and planning for the seamless transition to Harris for a few weeks now. That is probably why the assassination attempt on Trump didn't really tamp the situation down. The wheels were already in motion. That's why everything's moving lightning quick. That's why all these endorsements are flooding in.
This was all organized. We saw glimpses of it when Jeffries and Schumer told the DNC to chill on the virtual roll call.
Apparently, the President's final decision was made yesterday, but everything that had been said and was being discussed was being very, very tightly confined to the President's innermost circle. Even top campaign figures were blind-sided by the announcement and not told until - quite literally - one minute before the statement was released. This explains why we've seen campaign figures making the statements they have Friday, Saturday, and even as recently as this morning.
But - and these are sources, of course - Pelosi, Schumer, and Jeffries started this project shortly after the debate, and it wasn't even a question.
My speculation: They were ready if an incident such as the debate happened. I think they knew something about his health, that it wasn't as ok as advertised, so that they had fingers hovering over the abort button if things went south before the convention.
Because they moved quickly.