Majority of the people living in Miami are Cuban or Venezuelan. The Venezuelan population has increased a lot in Florida while the Cuban population hasn't really seen much growth in a long time now (immigrants).
The Marlins nearly go after Cuban players specifically because of this. Unfortunately the Marlins have been ran so poorly even with their WS wins it hasn't helped their attendance. I always thought the Rays moving from Tampa to Miami would have been a better option and then moving the Marlins to Nashville, Charlotte, Raleigh or Portland made more sense than keeping the Marlins in Miami.
The Marlins are basically a toxic franchise at this point.