Primarily the second. Biden barely won some key swing states (look up the Electoral College and all the bs that brings into US elections) in an election with higher turnout than any in the past century, 4 years of Trumps general fuckery in the immediate past, and an ongoing pandemic. (tbf, the pandemic should have been in Trump's favor, but he's Trump, so...yeah)

Public memory is short, and along with very recent stumbles and valid criticisms of certain positions of his, Biden is coming into this election very much on the back foot, even among his base, while Trump is going strong as ever. He needs every vote he can get and he's not getting anywhere near the support he had in 2020. It's looking pretty dour overall.