You are incorrect. Trump can and should argue ALL the facts. First that the whole trial is unconstitutional. Second that his speech in no way incited an insurrection. And lastly according to the article of impeachment part of the incitement was his months of “lies” about election fraud. So yes he needs lawyers to argue that there are facts that such election irregularities did occur and he was not lying.
Election fraud is so far into the weeds on this it should not be the primary focus of the case. I suspect he wants to be vindicated. He is fixated on it. He likely does want it to be the primary focus. It is not in his best interest for this particular trial.
The lawyers likely couldn’t convince him so they decided to part ways. Sometimes you have to cut your lawyers and/or the lawyers have to cut you.