Any reason for this
Well, Muay Thai is sort of over-rated. Most people believe that if you know Muay Thai, you know everything there is to know about stand-up fighting. And if you know BJJ, you've mastered grappling. Of course, more informed people know better. Muay Thai and BJJ are so popular because they are applicable in the ring - they aren't ideal for self-defense, contrary to what some people believe.
Muay Thai can be used against multiple opponents , but most fights almost always end up with the fighters (in street fights) struggling and grappling with each other (standing up for a bit, and then on the ground). The closest thing in Muay Thai to grappling/joint locks/wrist locks/etc is the clinch. The clinch is really useful, but it takes too long to take an opponent out (in a street fight, you'd want the fight to end quick), and I would think it'd be hard to get your opponent to clinch. Also, the quickest way to end a street fight, or get control, is to execute joint locks/wrist locks, which in Muay Thai you learn none of. Groin kicks, palm strikes, eye-gouging, or anything illegal in MMA tournies is not taught in Muay Thai, but these would be ideal in street fights, where anything goes.
Of course, the fact that Muay Thai utilizes knees and elbows makes it a really dangerous and effective art, but not ideal in all situations. Sometimes, and I'll probably be killed for this, Traditional Martial Arts have more potential on the streets. Hapkido and Kenpo would fare pretty well on the streets, in my opinion, if the practitioners train properly and realistically.
PS: I'm not pretending to be an MA expert. I'm a geek though, so I've read up on a LOT of things regarding the Martial Arts and this is what I know (or what I think I know) about Muay Thai. Hope I helped.