This info graphic is from a 538.com piece a few years ago (link here:
The NFL Should Expand To London. But First: Canada, Mexico And LA.)
This is why LA can afford all of those teams. It's the only metropolitan area in the ballpark of NY in terms of population. Even Chicago, #3, is less than half of NY. And based on their analysis, it still is #2 in estimated NFL fans despite their not having been a team there for 20 years.
I disagreed with much of the article itself because it focused solely on the numbers and not logistics (seriously, how do you have a team in London with current travel technology? How can you have a team in Mexico City based on the political and economic climate there now?). But this chart itself is great in showing the potential market size.
Someone mentioned San Antonio - it may be the 7th largest city in the US, but it has no metro area. Compare that to Boston which is practically all metro area (only 650K in the city itself). The full metro area is what's generally taken into account - for TV eyeballs.
Also note that the SF/Oakland population isn't that different than Boston, Philly, etc. It's not a surprise to me that the 2nd teams in that market (both baseball and football) are having trouble -- same as how the 2nd MLB teams in Boston, Philly, and St. Louis did in the 1950s and earlier. The San Diego market is very big. I wouldn't be surprised if (after the Chargers agree to move to LA with the Rams) the Raiders eventually wind up there.