Probably because you're mixed. And because it shows (I'm assuming, based on what little I have to go on from your post). I'm white American, so I'm a mutt, of a few different European lineages, but you couldn't tell that from looking at me. All you could tell is that nature was cruel elsewhere.
Racial purity is a big thing among a lot of different people, me included, I'll admit. Though I will add the caveat that I'm aware that as an incel, it really has no real impact on me. And in my day to day life, I do treat it that way, I really don't make a big deal of it IRL. But racial purity seems to be even more important in highly nationalistic and/or xenophobic areas. I don't know how nationalist/xenophobic Thai people are on average, but if the percentage is high, that's probably why.
Now, why this doesn't seem to impact Japanese foids very much, I don't know, as Japan itself seems to be very xenophobic (correct me if I'm wrong). I'm just guessing based on past observations, is all, and providing what I know, or rather, suspect. Make of it what you will. Always remember, there are many factors that lead people like us to be incel. As much as many of the people on this forum would like to believe otherwise, it does not boil down to just one or two things. It is a combination of many factors, and not all factors are equal. Nationalism and xenophobia could be two factors in inceldom that could both vary in significance geographically and politically.