That would be the doofus explanation.Bolton said at the time he would testify if the senate had issued a subpeona. Like most in the GOP, your Senators, Ted and John, didn't want to hear him. If you are like most in the GOP, you would have sought a good primary opponent for them if they had not protected us from the truth about Trump.
Anyway, Bolton's testimony would have made Tump's protestations of innoncence down only a little from the 1.2 to 1.0 - 0.8 on a 10 point plausablity scale. Anything about 0 is fine for his loyalists. In fact numerous folks, including some on this thread, are just fine with him using his office to demand personal favors. I think Bolton correctly surmises that this impeachment taught Trump a lesson... that he can do it and get away with it.
The truth would be that the Democrats were talking impeachment before Trump took office, had no valid reason for pursuing impeachment, had no case once they started their buffoonery, and looked like fools during and after the failed process. Only an idiot would continue to push the purely politically partisan claim that Trump should have been impeached.