Because they are both terrible. I mean, don't get me wrong, if we have to choose between them, Biden is so much less harmful than Trump it's not even a choice, of course we need to vote for Biden in the general election if those are the only viable candidates. But Biden has actively opposed any climate measure sufficient to prevent the worst impacts of the crisis. Factually, Biden is actively trying to destroy civilization. He is a strong supporter of the antidemocratic and inhumane system of capitalism. He literally took away the basic human rights to organize and withhold labor from the rail workers.
Also, while this poll may give you the impression we need Biden to prevent Trump's return, it isn't true. Both the last presidental election, and this poll, are more referendums on Trump than anything. Manchin isn't an interesting spoiler to throw in - none of the dozens of voters who think Manchin is better than both Trump and Biden would care to risk letting Trump win by actually supporting Manchin. If the Democrats put forward a progressive candidate, they would do even better against Trump.