He was polling at about half of Clinton’s 2016 support, and many believed that his support was “soft” and that another candidate could easily overtake him. Given what happened in IA, NH, and NV, that wasn’t such an unreasonable assumption, and what saved Biden was his strong support among black voters, as well as many white voters deciding to vote for whoever the most viable “not Sanders” candidate ended up being. Had someone other than Sanders ended up being the main alternative to Biden, or had Sanders made fewer enemies within the party, I don’t think Biden would’ve rebounded so quickly and easily.
A couple days after Iowa on twitter I predicted the Sanders would not be the nominee for 2 reasons: Pissing off most of the party constantly and his campaign being incompetent. And well... that's kind of why Biden became the nominee. Those who couldn't reliably beat Sanders in the early states became irrelevant, and so yes, when Biden appeared to have a solid base of any sort, he became the primary choice of those who either didn't like Sanders or were ignored by Sanders' campaign for being too bougie (aka, anyone not already being on board with Sanders as of last summer).
