I listed 23 cities. Those 23 cities probably represent 35 million people or more. That is 1/8 of the US population. That is just a small sample of the cities with no west coast service. I would bet that over 100 million Americans have no nonstop to SFO or LAX on United.

on top of that there are several large cities which have only 1 or 2 flights to SFO and those may not have very convenient connections. As someone who has lived in various midsize cities in the east and midwest I can tell you that SFO adds a connection for many places.

I don't know the answer but I would not be surprised if SFO has less than ten cities served east of the mississippi.