He always seemed to be the very centrist figure. I don't think he plays particularly well to younger, more progressive voters, but as has been acknowledged the alternative is Trump. I mean there's a false dichotomy that exists that it's Biden V Trump in such situations, but really it's a Biden V not voting. The quite detestable nature of Trump probably pushes people who aren't particualrly keen on voting for Biden to vote for him, in a manner that may not have been apparent for Hilary.
The other angle in this, is that sections of what you might call centre right republicans may vote for him, or dislike him too little to want to vote for Trump. He's a fairly innoffensive character. Obama (black) and Clinton (wife of Clinton, a woman) or Sanders (socialist) likely mean they are mor emotivated by Trump. Bidens a bit of a pickle, he's an elderly, white quite moderate man.That seems quite appealling compared to a lunatic in Trump.Apprently the Tea Party have not been as enthuastically for him as in other elections.
These are the dynamics. As I put yesterday, I think the conference season doesn't help him much. It probably re-enforced to people Biden is moderate and capable, while Trump is unrealiable and quite hateable. I don't see Trump really having the discipline to dial it down,appear a bit more restrained and pressuring Biden to take risks. It's not in his nature.