It's bigger than New Orleans or Buffalo plus it is growing very fast with a young population (avg age 35) and better than average personel income.
Maybe pro football isn't the game for that town, or maybe the stadium sucks, or maybe their marketing sucks... I don't know. But it has the demographics to support a pro team.
Then again, pro football couldn't do well in LA, the 2nd biggest metro in the US.
I don't know anything about it's demographics, but at least according to the FCC, Jax is 58th largest market in the United States. New Orleans is 45 (with the football nutty Baton Rouge, 87, only slightly more than an hour away). Buffalo is 53. Several other factors favor football doing better in Buffalo than Jax: Buffalo has a long and storied history of professional football, Jax does not. Buffalo has nothing else going on in the winter; Jax does, thanks to Florida's warm weather.