There's a big dose of unknown when it comes to evaluating who is a strong candidate in general elections. No one really knows before you run the primaries, and even then it still isn't very clear. Joe Biden ran in the democratic party primaries in 1988 and in 2008. In neither of those contests did he get any real traction, but both of those were before he was Vice President.
Coming into 2016, the Democratic party establsihment (namely Obama and his closest allies, and then probably some leftovers from Bill Clinton's troupe) wanted to run Hillary Clinton. She had come close enough in the primaries against him in 2008, and they also wanted to break the glass ceiling of electing a woman. I also think think they underestimated their opposition when they saw the Republican party in disarray given the number of candidates in their primaries, and didn't examine their own choice of candidate thoroughly enough.
Biden has always wanted to be President, and some stories have said he wanted to run in 2016. But it was apparently Obama who didn't believe in him, thought it was possibly too much for him to go through emotionally given the recent loss of his son, and so continued to move the party apparatus to nominating Hillary (some more background in this piece:
https://www.politico.com/news/magazine/2020/08/14/obama-biden-relationship-393570)
So I think that's kind of it. As I said at the beginning, no one has that great of a grasp of how naturally popular a candidate is or isn't at least until the primaries get going. And in 2016 Biden had essentially what you could call "his own team" in the democratic party establishment, supporting someone else.
It wasn't that anyone was sure that he was a weak candidate*. The 2016 results, the 2020 primaries, and so far at least the 2020 polls have seemingly proven them wrong about his popularity and chances as a candidate. If they knew all that in 2016 I imagine they would have made a different decision, but it was just unknowable.