Miami is a big and diverse market that is highly desirable for its corporate connections and market diversity.
As for Fusion, in 2001, anyone who followed MLS then knows that the Fusion, for all their perceived attendance issues were as good or better than other MLS markets. Essentially Kraft, AEG & Hunt picked the markets they wanted to save. It's just circumstance that both Florida markets had targets on them. Tampa was league owned, easy target. The owner of the Fusion wanted out, so another easy target.
Considering the changes in MLS and Miami in the 10 plus years since I think the Fusion would have been successful had they stuck around.
As for New Orleans, too small and too poor.