I have maintained that the polls have no validity for Biden when there is actual voting. It is my belief that primary voters intending to vote for Biden, are casting a different vote when the actual moment arrives, for a variety of reasons.
As it comes closer to the time to vote, voters start doing Internet searches on the candidates they are considering (if they are undecided, which many Dem voters are in this primary cycle).
They then find an endless parade of Biden gaffes and gropes. If they dig deeper they find out about the Burisma bribery scam.
Then they vote for somebody else, _anybody_ else.