Could be because the guy she's accusing is running against the guy who bragged on tape of sexually assaulting women and stands accused by several dozen women of doing just that. They guy who was banging a porn star while his third wife was pregnant and who screwed around on his first wife with his second wife. Decades-old accusations by one person don't match up well against that!
There are several factors going into it. “Hilary’s emails” is definitely on everyone’s mind; and the allegation is probably (at least mostly) false.
The hypocrisy of “Left Twitter” and the likes of spending the past few years literally or functionally arguing for taking allegations at face value — including words and actions of Biden, himself — and then turning around and applying a totally different standard is definitely something to behold. I can only hope the party that generally stands for liberalism, due process, and evidence-based action is reminded of the importance of those values irrespective of moral panic or justified need for social change. A decrease in public perception of need for evidence when an accusation of *anything* is made, over time, will never benefit the less powerful.