I once read that NFL interest in Boston wouldn't amount to anything without gambling. Overall, gambling does account for much of the NFL's prominence.
Before the 1950's, believe it or not, pro football wasn't that big a thing in America. When people (especially those who didn't live in a city that had a team) thought of the start of football season, it was college football or the local high school team that the average American thought of. A total of 13 cities had NFL franchises in 1950. The All-America Conference folded after the 1949 season.
It was the explosion of television that helped change this. Most teams had lucrative local TV contracts in the early 1950's. In 1956, the NFL signed a contract with CBS for coverage of all league games. This created fans all over the country. The NY Giants won the 1956 NFL title, and Madison Avenue caught on. The NFL increased in popularity by leaps and bounds in those years.
As I have previously written on the Forum, the NFL wants an honest game. Even now, though they tolerate thug types, a player caught up in gambling would be booted out. The Corporations want their endorsers to be "credible," and the NFL knows that the fans want the product they are putting their money own to be honest.