I think this has to do with more of the transient nature of the populous. Retires, immigrants, 2nd home millionaires ect... It is virtually impossible to have rabid fans when your legacy is mediocre, too good, too very good, too sub par average at best too very good. Basketball is not the primary sport for those in Miami. The hierarchy is "The U", The Dolphins, The Heat and the Marlins.
In recent years, D. Wade gave them nothing too be rabid about, outside of the championship year, he has been hurt and or his team struggled to be a viable commodity in the community. Inserting Lebron and Bosh into the mix has given them short term success but I think seeing failure with those guys in close games have their fans being very reluctant to cheer.
I say all this too say, just like its team, the fans are bunch a front runners.