I don't know the 2 cities, but for the average person, there might be more to do in Miami, despite how you feel personally. Tropical after all. Maybe more things like the beach and nightlife attract the average sports fan anyway. Plus, I believe there are a lot of people in MIami who are not from Miami or even from the USA originally. If that is the case, they may not have the same attraction to football as the typical American, or the same loyalty to make them identify with a Miami team, or even the money to go to games in some cases. That is one theory a friend of mine who grew up in Miami has mentioned.
I also wonder if Florida, being a hot spot for college football teams, might take some of the fans Miami might have if they were more competitive and the Hurricanes and Seminoles were not during that time. I may be off on that, though, I don't follow college FB closely.