It's because you rarely hear anything from Biden, at least anything interesting. While Trump...
Biden really should've couple years ago chosen a successor to give his backing to run now.
As it is, I do believe he is still more capable to handle the office than Trump - considering general understanding of various things, without even getting to morality. And it is not entirely unrealistic view that Trump is most likely republican candidate and Biden has best chance of beating him. So making those assumptions, it's too late to back out now, he just has to keep hanging on.