Really I think it comes down to Warren having found and almost completely cornered the market of “progressive, not socialist” Democrat’s, of which there is a sizable chunk. Bernie has his 15% of diehard uber-progressives, Biden has his rock solid 20-25% of moderates and people that aren’t paying close attention, and everyone else has had to try to see what they can do with the other 60%. But in that 60%, there has seemed to be a pretty fine line for what they are looking for in a candidate, and it’s somewhere much further left than Biden but just short of Bernie. Warren has absolutely nailed that spot and has seen steady rising for going on 3-4 months now. For everyone else, especially Harris, they haven’t found what that sweet spot is (or are entirely uninterested in trying to get there). Booker actually probably is the closest to Warren in finding that area but hasn’t caught any real attention for it. Buttigieg and Harris both are a little too far toward Biden and not really committed to the all out progressivism, which I think has caused a lot of voters to lose interest because of a “what’s the point” type of thinking.
Klobuchar, Booker, Beto, and Yang will eventually drop out and that 10% that they have right now will be huge in determining how this race looks going into the meat of the primaries. If that 10% goes to Warren, things get real spicy, real fast. If any significant chunk goes to Biden, then gear up for some really horrible debates and a sleepy, terrible campaign season.