You know, when the Mueller report was being investigated, I was waiting to see if anything would come from it. And, while there was stuff in there that definitely looked shady, there really wasn't anything concrete that you could point to for impeachment that most people would understand. That is one of the reasons that Pelosi kept saying no to impeachment proceedings.
However...............when the Ukraine phone call came out, it sounded like there could have been something there. Then, Trump released the transcript of the phone call which showed that there was something that looked like quid pro quo. The whistleblower statement released the following day confirmed it. Since then? There have been things that keep surfacing from this investigation, and each day it looks more and more like Trump actually did something wrong, and that is why Pelosi finally launched the investigation.