There was nothing about Romney from his pre-2012 campaign days until comparatively recent times that you weren’t still saying about George Bush and Sarah Palin and are still saying about Trump.
I remember well the lies, the laughter, news reports that Romney never paid his taxes, and all the ridicule on the talk shows and from news anchors. Seeing Trump triumph over even worse treatment probably effected him.
Romney could not take the defamation, libel, slander, insults, and ridicule even at a time he had a good chance of winning.
Having failed, Romney wants to destroy someone who accomplished what he himself promised but failed to deliver, or had no such opportunity, and enjoy his moment in the limelight and the glory from the left and Hollywood and the D.C. Beltway, which is probably what he wanted all along, and did so at the expense and wishes of those who supported and paid him to represent them in the Senate.
Besides that, I’m suspicious of why he doesn’t want the Ukraine and Burisma Holdngs and the Bidens looked into—which seems to be his greatest concern these past six months, and the reason for half his vote.