Speaking in general terms, its likely due to low voter primary turnout. The change side of the party (the progressive side) is much more organized since they a

Speaking in general terms, its likely due to low voter primary turnout. The change side of the party (the progressive side) is much more organized since they are attempting to challenge the status quo and therefore have much more recently put in place infrastructure to get out the vote (perhaps peaking in response to the tech influx/peak BLM/Trump) to get Chesa in. The incumbents are generally properly larger but largely apathetic/too busy trying to afford SF rent - unless things go too far in the wrong direction. I think the recent attempts by the newly installed progressives at renaming schools (while the schools were closed for covid), changing the admissions to the magnet school to be non-test based, etc. - led to some infrastructure being built among the moderate, middle class family democratic voting base - particularly on the western side (Richmond/Sunset) which is mostly Asian. The fact that the media reported a lot of Anti-Asian crime in SF that Chesa didn't seem to do much about - probably fit well with this newly organized group.