Because they wanted him to.
This.
The only way you could have regarded the IA caucus + NH primary as more reliable bellwethers in the Democratic nomination process than the SC primary in the year 2020 or based your predictions of Biden's "collapse" on his showing in those two states while deliberately ignoring his firewall in SC (which was quite clearly always going to be his strongest early state, much like it was for Clinton in 2016) + not factoring in the predictable rapid narrowing of the field after each early contest and Super Tuesday (which clearly hurt Sanders) is if you let your personal feelings about Biden/Sanders or some (unnecessarily) overarching and wishful thinking about the direction of the Democratic Party interfere with your predictions. It was fun while it lasted, though.
