I've trained Muay Thai in Europe and it is quite popular in countries such as the UK, Spain, Netherlands, and Poland. When I went to Thailand for training, I expected that I would meet a lot of guys from Europe and also from the US but, I mostly met guys from Europe, the UK especially, and not many Americans. I thought that the popularity of MMA would introduce Muay Thai to a wider audience in the US, but it seems that it didn't. Or are there schools in the US that teach Thai Boxing, and it's just not popular to train in Thailand?
In the US, training in Muay Thai and training derived from Muay Thai is popular as part of mixed martial arts (MMA). Every MMA school pays at least lip service to having Brazilian jujutsu and Muay Thai training, and MMA is very popular. MMA competition is far more popular than Muay Thai competition; people may train Muay Thai, but it is often only for their MMA striking game, not to compete in Muay Thai competitions. With this preference for MMA, it does not make a lot of sense to travel abroad; MMA is already in the US.
I think that the primary difference you'll see is that Muay Thai seems more likely to have been subsumed under a more generic "kickboxing" or "MMA" label in the United States. I do not have any data on-hand, but anecdotally, I always see Muay Thai listed as a subset within the "ring combat" martial arts which tend to be trained as one unit without any of the cultural ceremony.
To a lesser degree, this might also be the influence of cinema. Some of the most popular martial arts in the United States such as Karate and varieties of Kung Fu have been fueled by the movies that are out. Kids watch The Karate Kid, or a Jackie Chan film, and they get their parents to sign them up for the respective classes. Some kid watches Only the Strong and decides they want to learn Capoeira. While there are definitely Muay Thai movies out there (Tony Jaa's output for example), they tend to be more recent, or were subsumed under a more general heading, as with the JCVD Kickboxer series.
Lastly, it might be a self-feeding problem of that martial arts styles tend to be championed by locals who have learned the art and wish to spread it. Take a look at Chuck Norris and how he spread Tang Soo Do throughout the United States, or the various Karate or Judo competitors who went out and started their own schools. Because few people in the United States train in Muay Thai exclusively, fewer come back to the States and open their own school, which means fewer kids are exposed to it.