Actually, the Rams are originally from Cleveland — but that was a LONG time ago! (Just as the Redskins are originally from Boston.)
The 50s were when professional football began to take off. Historians of the game credit television and a post-war masculinity for the increase in popularity. Maybe it was just that the number of players who played in high school reached critical mass 50 years after the game first started being a high school sport, and those players liked watching the very best.
Whatever it was, the league took off, and franchises became a stable business, creating rivalries that last to this day, and which are part of football's attraction. Tinkering with that hurts the game. There have been no franchise moves that I can recall that needed to be made because the team could not make money in a city. The Cardinals moved from St. Louis because everyone hated the owner who had no interest in winning, not because people in St. Louis wouldn't support football. Oakland moved the first time for greed, then didn't get a big stadium and moved back. The Rams moved to St. Louis because the widow of the owner wanted to be popular with her high school friends. I don't remember why the Oilers moved, but it can't be because of the town, since the Texans are doing just fine. Jacksonville may be a stretch as an NFL town, I don't know. But other than that one, I can't think of a single team that needed to move for lack of fan interest. Teams that never do well are supported year after year (Chargers, Detroit, Philly, Cleveland.) The owners are greedy bastards, and they are taking a big chance with the golden goose.