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?
Ladyboys is basically a term usually used for trangender in Thailand. ladyboys are men who dresses up as women. Ladyboys are very pervasive in Thailand. They can be found in stores, schools, clubs, and many others. They are regarded as a third gender in Thailand. Transgender is very popular among Thais because it is something they have accepted into their culture. Parents usually give their sons, who at the early ages start to play with dolls and see themselves as homosexuals, hormone drugs. Some boys, who are homosexuals, at a tender age take homone drugs without their parents' consent. Also, Theravada Buddhism - a form of Buddhism that is very popular in Thailand- categories gender into three forms : male, females, and kathoey( transgender). So, one can conclude that religion also conforms such acts. Ladyboys are not stigmatized in the Thais society, rather they are usually entertainers in bars, hairdressers and can be found in several occupational fields. So with or without little resistance from the society, the ladyboyism continues to grow in Thailand.