Why do Joe Biden’s issues matter, but Trump’s do not?
Because Democrats care about these issues and Republicans don't.
Republicans have demonstrated that they just don't care about supporting a creep of epic proportions, so Trump's slimy history isn't a problem for Trump.
Democrats, on the other hand, do care about this stuff, and finding out about Biden's problems may well diminish enthusiasm to get out and vote for him. "He's a creep, but at least he's not Trump" isn't exactly the kind of stuff that's going to energize the Democrat base.
-k