That has absolutely nothing to do with it.
First of all, as pointed out, there is no evidence LA wouldn’t support a team. We are, after all, talking about a city of 12, 15 million people. It’s not going to be hard to find 65,000 every Sunday to go see a football game.
Secondly, pro sports leagues do not rank attendance among their most important factors - especially the NFL, which could probably make a profit without ever selling any tickets at all. What most matters to pro sports leagues in terms of franchise placement is getting free stadiums. That’s why there’s a hockey team on the outskirts of Fort Lauderdale, and why Seattle lost its basketball team to Oklahoma City; the city that gets the team is, within reason, the one that offers the most in the way of free stadiums, revenue streams, and tax breaks.
You can’t really construct an argument that the Rams are doing any better at St. Louis than they did in LA. Rams attendance was usually pretty good; it cratered in the last few years in LA largely because the team was incredibly bad and was pissing people off on purpose, but prior to that attendance was fine. Attendance in St. Louis has not exactly been super, and of course St. Louis already lost a team before - again, it has to be noted the team is dreadful, so that has a lot to do with it, but still there’s no evidence St. Louis fans are any more dedicated than LA fans. The team moved to St. Louis entirely because no government in southern California wouldn’t give them hundreds of millions of free dollars in the form of a shiny new stadium. The Rams didn’t actually care WHERE they moved to - St. Louis wasn’t even their first choice. They explored Baltimore first, but went with St. Louis when the city promised them millions. The NFL was not happy about it, actually, as they knew full well there was nothing wrong with the LA market, but couldn’t stop them.
It’s a misconception, I think, that pro sports leagues try to put their franchises where the fans are. They don’t, beyond a certain point, or at least that isn’t the only thing. The NHL has as many teams in Florida as it does in Ontario, which if you’re trying to fill stadia with hockey fans is absolutely insane; if you’re chasing hockey fans, Ontario should have five teams, and Florida should have one. There was no reason based on fan availability to move the Seattle SuperSonics to Oklahoma City; there is no compelling reason why Memphis should have a basketball team but Cincinnati or Kansas City do not; the continued presence of the Tampa Bay Rays in St. Petersburg is clearly not aligned with getting people to come to the ballpark, and if you want another NFL example, “Jacksonville Jaguars” is all I need to say.
All of these things CAN be explained; stadium deals, tax breaks, attempts to grow TV markets, and things that were well intentioned but just did not work. But they aren’t aligned with where the fans are.