Biden’s polling in the 20s, occasionally breaking into the 30s, and he's running low on money. Sanders and Warren have much bigger warchests, and there's a lot of votes to be picked up as minor candidates fall off, which it'll be hard to compete for without funds. He's in a weaker position than the early frontrunners that ended up not winning in basically every Democratic primary in a generation without an incumbent President or Vice President in the race. (EDIT: Yes, 2016 was an exception to this, but Biden has nothing like the DNC backing Clinton had, nothing like the office-holder backing Clinton had, and nothing like the DNC rules that magnified the effect of both of those advantages.)