I think it's a matter of perspective as well. America has great wrestling - it's ingrained in the culture of America in the same way that boxing is in Mexico and my home in the UK. Asian martial arts are scene as 'self defence' or a hobby in the west, there's something foreign and different about it.
In Japan, for Judoka, it's their life, Korea has university systems that train you up in Judo from nothing and can eventually get you a degree in Judo to pass the knowledge down. I think there is a fundamental disconnect in what karate, judo, kung fu etc. mean for us as westners as opposed to what they mean for people in Asia.
In Muay Thai, while the martial art is quite old, it's exposure to the West is quite new, but in the west we live fundamentally different lives. People in Thailand don't really do Muay Thai as a hobby, they either don't know it at all, are a fighter, or they were a fighter and now they've retired. It's not really something you just do for fun - like we do. I think it's that difference in how we live vs how they do.
I don't necessarily think Americans lack discipline, the wrestling culture is fantastic and I think Americans are very hard workers - but I think that whenever you borrow a sport or an art from another culture, it's very hard to practise that in the same way that culture would.