Biden had obvious weaknesses. He was not an inspiring speaker, he was cantankerous, he bickered with voters, and his Senate history raised big red flags, which Kamala Harris dared to bring up at the early debates.
But you know, if you're someone like Julián Castro, for example, you don't stand a chance against this guy no matter how great a politician or candidate you might be. Especially when a returning Bernie Sanders is sucking the rest of the air out of the room.