this question is like asking why do some dudes like thai or asian girls... and i really don't have an answer for that. they're just what i'm attracted to. i caught yellow fever in the 90s, and i haven't looked back since.
i do know that physically speaking i like girls with dark hair, dark skin and dark eyes. conversely, blonde haired, blue eyed chicks do nothing for me.
and i imagine it's the same for thai girls. some of them like whites, even a certain type. and others may like latinos, etc, etc...
but not everyone is like this. by and large it seems *most* people tend to stick to their own kind when looking for a mate. it's not as if all thai girls are looking for farangs.