As always, the polls generally do not track specific candidates amongst likely voters, they just randomly call people and get a popularity vote.
Biden is more popular right now because he's been in the basement, he will most likely win the popular vote as long as he doesn't speak, but that's not how you pick a candidate. The minute Biden enters a podium and thereby the media spotlight, the real numbers will start coming in.
Given Biden has not yet picked a VP, you can't track the Biden ticket yet because it doesn't exist yet. Even if he wins the popular vote, he has to win the state votes and there a Biden win is very unlikely unless he flips pretty much all of the swing states which voted overwhelmingly for Trump last time and Clinton was a more potent candidate than Biden, especially in the demographics where it matters.
Biden's core demographics:
- Blacks - have been disillusioned by the establishment Democrats and more and more conservative black voices are coming up.
- Progressive young people - have been disillusioned as well but for different reasons, Biden isn't Sanders, he's not far enough to the left, young voters will stay home.
- Soccer/security moms - Defund the police won't be popular policy there
- Suburbs - White flight and Democratic strongholds polarization is having a very strong effect on the demographics and interests of the suburbs, turning typically blue districts more and more purple. Defund the police won't be popular there either.