Well. It can be done on "tall catamaran". Technically speaking, there is 7 miles of shallow water and channels only. Florida Bay can be by-passed - sail to Marathon and take intercoastal from there if conditions are not right. It would add 20 extra miles only.
I believe, problem is in really rougth conditions, typical for the challenge, and non-stop way of doing it. There is a limit of how long you can sail high performance cat in "interesting" conditions. Slower and more comfortable boats can stay out longer and cover greater distance at the end. Smaller and more maneuvrable boats can take less exposed routes. Plus that great "NOWHERE" called Everglades takes is toll too - you are so far from any civilization....