Well, it all comes down to Trump. He won last time, in what was really actually a surprise to many. He pulled that off, also, seemingly surprisingly, not in spite of but in large part due to a lot of the things people assumed would hurt him. He was dodgy, he was brash, he was inappropriate and he had a very checkered past.

He seemed to Intuit exactly how these seeming electoral disadvantages would actually play very well and certainly even continued to help after he won. But to at least some degree this did start to catch up to him somewhat over the course of his presidency and to a limited degree his opponents were able to impose some consequences that seemed to hurt him at least a little, notably being twice impeached. For all their bluster about "nothing burgers" this did seem to actually rattle his supporters as it was to some degree a way of objectively nailing down that he really was actually a bit of a crim and it wasn't all just self-created bad boy mystique and wasn't just character assassination. For all they might say about the fairness of it, he was undeniably impeached, twice.

This seems to have been a festering sore spot for his supporters, especially in the wake of his defeat and now unofficial re-election campaign. Now it's no longer so certain he can pull off the same magic trick twice and turn his flaws in to virtues, it's imporant for his campaign to try and level the playing field. They need to try and change the equation where on the one hand you have one candidate with a shady background whose known to have been impeached for their dodgy behaviour, and one who hasn't. It's going to be a tall order even for "alternate facts" driven campaigning to somehow elevate Trump to equal footing so the better strategy is to try and find a way to drag his opponent down instead so that now, for anyone who might care, you can say "look, they all do it, they're just as bad as each other in this aspect.l, and besides when it happened to our guy it was rigged and unfair '. It muddies the waters just enough. Also if they get any traction with this, if they impeach Biden, it's a tough spot for Biden's camp for an additional reason because if they defend on grounds of some sort of unfairness about the process, well, logically the whole impeaching thing must be pretty meaningless then because either all candidates are dodgy to some extent and it doesn't matter or the impeachment process is so partisan that it's conclusions are irrelevant and Trump doesn't look so bad anymore.