When Biden first announced and was far, far ahead, I predicted that Biden would not be the nominee because I have seen him for 40 years. I gave 4 possible reasons.
1) Biden would step on his d!ck one too many times, meaning he would say something really, really stupid. He is a human gaffe machine. They are trying to hide him like they did Hillary for a different reason.
2) His health could go bad. Just a possibility because it is a grueling grind for these guys, and he has a few years on him.
3) He gets caught up in Russiagate/Spygate spying on Trump. If they can prove he is connected once all the facts come out. I don't know if he is or isn't involved.
4) He/his son's connections to Ukraine/China.
All 4 of these are still on the table. Could be more than one of them take him out before it's over.