Generally speaking their reasons were that Trump was a horrible person. Also in general terms they seem to be people who talk a lot about love being the dynamic of the Universe and espouse subjective concepts such as unity and tolerance and inclusiveness as if these concepts are virtues. Honesty is a virtue. Unity is not. Patience is a virtue. Tolerance is not. But even though in the moral realm they are confused on these concepts of virtue they think of themselves as "good" people. Which is not abnormal as even convicted criminals will tell you that they are good people. OJ Simpson thinks of himself as a good person and told us so.
Being good and promoting love as the dynamic of the Universe is, in and of itself, a noble idea. But practiced without discernment and wisdom when it comes to choosing a leader who enacts policies for ALL Americans could be tragic. Love is a beautiful thing but tough love is far greater. Enabling an alcoholic who is killing himself and destroying those around him by being permissive with the alcoholic in the name of love and tolerance, for example, in the grand scheme of things, is dangerous, foolish and cruel.
I won't elaborate on Biden's reign as the disaster it is for American women, workers, diabetics and the military because that would entail a policy analysis and that's not my point here. My point here is the gross disparity between the claims of love and goodness and concern for humanity expressed by Biden supporters and the horrible results his administration is already enacting and signalling.