There are quite a few reasons why Florida does not vote so strongly Republican.
1. Senior citizens, many of them Jewish.
2. The Miami area, although not as strongly Democratic as one might assume (due to Republican-leaning Cubans), still strongly helps the Democrats.
3. Southern Florida is not really all that "southern" to begin with.
Dade County had 861 people in 1890 (and this was before the areas that became Martin, Palm Beach and Broward counties) were split off.
By 1900, this increased to 4,955.
By 1910, Palm Beach had split off and the combined population was 17,510.
By 1920, Broward had split off and the combined population was 66,542.
By 1930, Martin had split off and the combined population was 219,941.
1940: 393,817
1950: 701,512
1960: 1,514,031
1970: 2,264,680
1980: 3,284,858
1990: 4,157,000
2000: 5,139,295
2005: 5,561,928 (estimate)