Well, it could be costs. There's a thread right now over at the airliners.net board where LCCs flying international is being discussed. It was pointed out that while JetBlue and Spirit fly international, and JetBlue has apparently flirted with Canada service, that only WestJet (Canada's version of JetBlue) and Frontier are flying now. Neither WestJet or Frontier on Canada routes, or JetBlue and Spirit on Caribbean routes, are charging fares as low as AirTran (in general, of course). Someone pointed out in that thread that Southwest and AirTran have two of the lowest cost structures in the industry and avoid international flights almost like the plague. My personal note on that is that they also avoid codeshares like the plague, while the slightly more expensive airlines like Frontier (codeshare on Essential Air Service routes) and JetBlue (codeshare on Cape Cod flights) have picked up on this a bit more.

In other words, it seems like AirTran and Southwest are doing everything possible to keep costs down. JetBlue tends to have slightly higher fares in my experience, Frontier definitely has higher fares for every flight I've ever looked (granted, not originating or ending in Denver), and Spirit nickel-and-dimes customers. I guess it's different philosophies, but international flying is definitely more expensive because of the additional regulations, taxes etc.

I still think AirTran should be in the Caribbean business though...a very good market for them, even at higher fares.