What a difficult question! I have thought about it quite a bit myself, and although I don't have a direct answer, I have a theory.

I think the main reason for so many "ladyboys" in Thailand, is the fact that the country and its people are quite open to the idea of it. They also don't seem to flinch at a gay or lesbian couple, unlike people in many other countries that I've been to. Of course I'm not saying that everyone accepts it equally, but they don't seem bothered and assume the "live and let live" stance on things.

Another reason I can think of is the fairly low cost of quality treatments and surgery. But hey, I guess we can't be 100% sure, can we?