A lot of times, it's because they learn early on that they don't appeal to Thai men or that Thai men treat them like <deleted> by bullying them. Especially the dark skin girls. Thais men can be quite racists and the idea that dark skin is ugly is real here. Same for some facial traits deemed unattractive for girls.
Another reason is if the girl is a single mother, Thai men will not touch her anymore.
Money is ofc a main reason. Almost all women around the world are looking to date up and to be taking care of. This is especially obvious in poorer countries like Thailand.
Some are just attracted to differences, and have been disppointed by Thai men and like foreign men.
Some have a big penis fetish.
... it's different for everybody.
Thai girls mainly date Thai men however and have zero interests in foreign men because why would they go out of their way to learn English, and try to form a relationship with someone not sharing any cultural similarities or references with her?
In my experience here, almost all Thai girls i've dated were all looking to leave Thailand one way or another, for job or travel. Most of them did, sometimes with the help of foreigner, sometimes by themselves.