One would be hard-pressed to think of a language as difficult for a Western person to learn as Thai.
1) It's a tonal language.
2) There is no universally standardized way of even writing the alphabet in English so that a foreigner can try to pronounce it.
3) Tonal languages use both cerebral hemispheres; non-tonal languages only use one. So it's technically easier for everyone the world over to learn non-tonal languages than tonal languages.
4) Dialectical variations across the kingdom.
5) Thai media probably does not portray Westerners trying to speak Thai as often as it ought. Because of that, the Thai ear is not accustomed to what that particular train-wreck sounds like and so can't easily understand what's being said when they encounter a farang trying to speak it in the wild.
6) Like it or not, Thai is not an international language like Mandarin. There aren't many university courses that offer it. And because of that there isn't as much global effort put into providing Thai instructional materials as there is for other languages. Thai is still very much a niche language on the global stage.
7) Thai learning apps are not as good as the apps for other languages.
8 ) Not only is Thai a tonal language, but its list of vowels and consanants is different from that of many other languages. Those who try to learn Thai must learn how to, effectively, make sounds they've never made before.
And all of the effort that that demands is just to learn the language of one single kindgom that, despite its other virtues, still does not amount to much on the global stage in comparison to some of its richer, older neighbords; a kingdom that, wrongly, has an unfortunate international reputation as a throw-away locale for tourists to visit, abuse, and then discard.
Learning Thai takes a lot of effort and a lot of time for what amounts to relatively little pay-off. It's unfortunate, but true. I am presently on my third attempt to learn basic Thai. If even after this, nobody can understand what I'm trying to say, I will give up. Many other foreigners would give up after the first go. Perhaps the most difficult pill to swallow -- a pill I don't anticipate being swallowed any time soon -- is that if Thailand really, really wants to become more culturally accessible, there will need to be a nationwide movement away from the Thai language and toward either Mandarin or at least English. Thai will never be a global language, even though the Thai people really ought to be globally respected and listened to.