TV helped, but it wasn’t the predominant reason - there was no competition. If the Miami Dolphins were charging $120 for basic seats plus $20 parking and $15 hot dogs and the Miami “Hurricanes” in an alternative league were charging $40, $5 and $5 for the same thing, which do you think most would go? Same thing at the TV level. When you are the ONLY game in town for that, you can charge big bugs.
The lack of competition in a single market (in Miami, to cite your example) isn’t the result of monopolistic practices by the NFL, though. The NFL simply has a stronger brand name than any other competing football league, which means that the best players want to play for NFL teams, and the fans will pay more to see them.