The geography makes Florida a peculiar area for radio. First of all, with the exception of the Tampa to Daytona Beach corridor, the population tends to hug the coast. You wind up with markets that are 20 miles wide but 150 miles long. The state is very flat. You can stand on a brick in Miami and see half way to Orlando.
The Keys are in a similar situation with buckets of FMs that can't cover the county. There are only about 90k people in Monroe County and very limited non-tourist related businesses as an advertising base. As in some places in the country where you have "hobby farms," Key West has more than a few "hobby radio stations." They loose money and under cut each other on the rates but the well healed can say "Hey, I own a radio station in Key West and spend every January to March in my condo to run it."
It's also very expensive place to live. Housing costs are through the roof as is auto insurance and home owners insurance.
I am not sure whether the ethnic diversity in the Miami market makes it harder or easier for stations to sell advertising. As I tune around you hear tons of religious stations in English, Spanish and Creole. Between "regular" stations, translators, LPs and pirates the entire FM band is full.
All-in-all, there is room for only a few stations to make money.