I am sorry, but I do not buy this theory. What about places that received a franchise for the first time or had not had a franchise in years? The population in those cities seem(ed) to gravitate towards their new team.
There are very few places like Los Angeles in the United States. It's a huge metropolitan city where there is something big and huge happening all at once. This isn't a city that shuts down or focuses on one team or event. Many of those cities that received a franchise for the first time didn't really have any other pro teams, and if it did, it's likely one or two existing pro teams. Los Angeles has the Dodgers, the Lakers, the Clippers, the Kings, and then UCLA and USC. It also has the the Galaxy and LAFC, on top of that, there are the Angeles and the Ducks about 45 minutes away (depending on traffic). Oh yeah, there's the Chargers too. With the exception of a few of the teams I listed above, most have a long history and tradition of winning. In addition to this, we are one of the entertainment capitals of the world. Hollywood is here. The studios are here. Disneyland is here. Universal Studios. The beach. The mountains. EVERYTHING is here. In order to compete for people's attention, the Rams can't just come to LA and expect a following. They need to earn it. It's not fair, but there are so many things to do in Los Angeles in any given weekend -- and on top of that, Las Vegas is just a short 4 hour drive east and San Diego is 2 hours south. Oh, want to go to Yosemite? That's only 4 hours north east. Want to go to Santa Barbara that's only 2 hours north west. I'm from San Francisco and even I didn't understand how VAST the Los Angeles metropolis is until I moved here, and the unlimited amount things to do here -- on top of that, the weather is always spectacular that you WANT to be active. You want to be outdoors and play in the sun. There are so many things to do here. If the Rams are just okay, it's not going to capture anyone's attention. It's a great thing that they've done so well in the past 2 years and they have gained a following. People aren't rabid for the Rams, but the fanbase is slowly growing. It will just take time. It's moving in the right direction (this season is a tough one), but the fanbase is starting to care and going to a Rams game is becoming an attractive option for many people who are slowly becoming Rams fans.