one reason the impeachment might not happen: it would be a YUGE distraction just as Biden is kicking off his presidency. he's talked a lot about how much he & congress need to get done on day 1, but impeachment would suck all the air out of the room. it would delay the confirmation of Biden's cabinet picks, delaying every agency's work.
and predictably, trump supporters are going to cry foul at the speed of this impeachment. "it was rigged from the start, he never had a chance to defend himself!!!111".