I don't know if you'll be able to answer my question, but I'll give it a try to see your insight. Why do you think that is the reason? Are most of the programs affiliated with some Christian organization?
Sorry this is a late response. I'll try to give you an elaborate answer.
Some international schools in Thailand are basically Christian schools. The one I went to is called EIS and its an SDA (seven day adventist) school. In this kind of school they normally try to recruit teachers from universities affiliated with SDA beliefs first. I have seen non adventist teachers but they're so rare and they're normally white.
On most other schools, its important for your skin to be white. Thats because they have an image to uphold. I think Bangkok Patana HS is an Australian based school so they would normally hire Australians to teach there. One of my teacher was fired for swimming on saturday (LOL) and he got a job there. He's a white Australian. Now he is making alot of money.
For some reason Thais just think that only a white guy can speak a white language. Its just the way it is. BTW if you're a black guy.. good luck finding a teaching job there lol. At least in prominent schools.
Anyway if you don't believe me just look at the teacher lists (if you can) of top international schools in bangkok. Its mostly all white.
I mean obviously there can be exceptions to this rule, but lets not kid ourselves.