Some Republicans voted for Biden despite having no leftist beliefs and not really liking Biden that much, simply because they really didn't like the direction that Trump had taken the Republican party. They felt like their party was moving in the wrong direction, but they didn't actually identify with the Democratic party, either.

I think that the reverse would have happened if, for example, the 2016 presidential election had been between Mitt Romney and Bernie Sanders: probably a lot of moderate Democrats would have voted for Romney, as Romney is more similar to Obama (the previous Democratic president) than Bernie Sanders is/was.

Nowadays, people are a lot more partisan (understandably), and I can't see why a Democrat would ever want to vote for Trump today.