Tropic Air and Maya Island Air cannot fly to Mexico, for the same reason they cannot currently fly to Guatemala. Belize does not currently meet International Civil Aviation Organization (ICAO) and FAA safety oversight standards and is listed as a Category 2 country.
Category 2 airlines cannot fly into Category 1 countries, including Mexico, Guatemala, Costa Rica, the U.S. and so on. Honduras is a Category 2 country, which is why for a time Maya Island Air was able to fly to San Pedro Sula. (Believe that service has since been cancelled.)
--Lan Sluder
AmbergrisCaye.com 
