Part of the reason we see so much negative news about Biden is that it's more newsworthy. Compared with Trump, discovering something bad about Biden is a real scoop.
Trump is one negative thing after another. There's nothing new to learn about his badness, and no new development, however awful, would make him significantly worse.
Biden, on the other hand, has had a long and mostly distinguished career. Bad news about him - his debate performance, his son's troubles, etc - stands out.