Because how does it look when the party that became the champion of the Me Too Movement and who used a sexual assault allegation as a reason to try to block Brett Kavanaugh's nomination to the Supreme Court. Then turns around and nominates and endorses a candidate that who also has been accused of sexual assault. Especially when the media hasn't really been in a rush to talk about the story.
While in the grand scheme of things given who is currently sitting in the White House, you're right those things shouldn't be problem. But for some Democratic voters who aren't too excited about Biden in the first place, this just gives them more reason not to be thrilled with him. Then there is also the question how do Independent voters feel about him.
At the end of the day, it all boils down to how much you are willing to hold your nose as you overlook some of the candidate's past transgressions and vote for them. Whether they're proven to be real or are they just allegations. Trump supporters have shown they're willing to do just that. Will some Democrats and Independents be willing to do the same. Speaking for myself, I know I'm.