I think the difference here is in the training methods. One one hand you have muay thai, grappling, etc. They are very focused arts. bjj has no weapons, no real striking, no long forms, etc. It's just grappling. You learn a new technique, then drill it, then spar with it. Beginners will roll as early as their first day of training. Consequently, they are testing themselves (to some extent) from the very beginning. Also, 100% fo their training time is geared towards grappling - they advance more quickly.
The same can be said about muay thai. bagwork, pad drills, etc. are all directly apply to the sparring that they will be using, and IME, they start sparring at an earlier stage than TMA, which makes a big difference.
One more thing to consider may be the types of people certain arts attract. As was pointed out earlier, people drawn to kung fu are "nerds". Of course, not everyone of them is a nerd, but how many of them are expecting vigorous training, several hours spent training in and out of the school, body aches and pains, etc.? If it were a great number, then the McSchool may not be so dominant today. BJJ, MT, boxing, etc. are known as being highly competitive sports. Naturally, you expect hard training if you are going to train in one of these styles. People interested in training sport styles are willing to endure that. The ones who aren't either adapt or quit.
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