I think the realisation is setting in, that it's beyond the realms of possibilities he can win.
Clinton v Trump was a 70-30. And some of the 70 for Clinton was based upon EC advantage, some incumbence bounce etc. Polls were quite volatile and close. Thats not the case now. It's increasingly an 85-15 and by election day Biden will be 90+ certainty to win.
This is like last seasons Everton. We might be up 3-0 at half time, but still know it’s quite possible to lose 3-4