An interesting question. I had a look on the web and found this site with a distribution map for Lythrum salicaria in America. I does seem to be missing from the extreme south-east. http://plants.usda.gov/cgi_bin/topics.cgi?earl=plant_profile.cgi&symbol=LYSA2
It is a cool weather plant from temperate regions, so possibly it finds the climate in Florida too much for it. It is a significant pest in a lot of areas in the US so it is probably a good thing that there are some places free of it.
It is a world wide species, and I have seen it in Canada, England and Australia and it is a native plant in all threee countries. I have one plant in my garden and it does not show any sign of spreading unduly. Still only the one plant after about 15 years!