He has long presented himself as being unfairly persecuted by his political opponents. Each new accusation is evidence of the persecution. Convictions are evidence of how extensive his opponents' power runs.

If this argument is true, then it is vitally important that his opponents be defeated. So it is no surprise that his supporters are extremely enthusiastic.

It's somewhat more surprising that his opponents are less enthusiastic about their own candidate. At least at the moment they seem to considering primarily his own actual time in office, which has been competent but not exciting (from their own point of view).

It's possible that, faced with the actual choice between the two in a year, they may decide to vote for him anyway. Meantime, it's not entirely surprising that the evidence of criminality makes Trump's supporters more, rather than less, enthusiastic.