Dean Phillips was a candidate who tried running against Biden in the primaries. I'll link to him talking about his experience in congress and how he felt that the Democratic Party retaliated against him for running: https://youtu.be/1hh8lcoJ1NA?feature=shared&t=1308
Pretty much. He ran in the primaries, they were mad at him, he lost his committee memberships and is no longer seeking re-election to congress, he speculates that this attitude from within the party is why no one else seriously ran against Biden.
I'm not alleging a grand conspiracy or anything. It just seems like internal party politics and personal incentives led to the democratic party ultimately ending up with a nominee that people aren't excited about (who I will vote for anyway).