It's because St. Maarten is Dutch/French, and does not recognize US cell phones. The towers are out there, but foreign (non-US) islands prohibit US cell phone plans because they do not make any money from them. The only US island/territories in the Caribbean are: St. Thomas, St. John, St. Croix (US Virgin Islands), and Puerto Rico (San Juan being the capitol). And th-th-th-that's it, folks! That's why the cell phone you purchased in the US, with a US plan, will work in those places, but nowhere else. ..usually. Here's a breakdown:
St. Maarten/St. Martin......Dutch/French
Grand Cayman.......independent, but formerly British
Bahamas...........independent, but formerly British (Nassau, private islands)
Aruba/Bonaire/Curacao...........Dutch (the ABC islands)
Cozumel/Calica/Progresso/Costa Maya..........Mexican (think Telmex card)
Jamaica...........independent, but formerly British
St. Lucia..........British (formerly French)
Martinique.........French
Barbados...........British
Dominican Republic.......independent, but formerly Spanish
Antigua........British
Domenica.......British
Grenada..........British
St. Vincent/Kitts/Nevis.........British
Guadeloupe.........French
Haiti.........(who goes there now?) independent, but formerly French
Belize.....independent, but formerly Spanish
Costa Rica (A+ in my book)........independent, but formerly Spanish
Anguilla........British
Isla Margarita.......Venezuelan
BVI (Tortola, Virgin Gorda, Jost Van Dyke)....pretty much British
If you'll be stopping at any of these ports, call your cell phone company ahead of time. It really depends on the US plan you purchased.