Actually if you read my posts, it completely revolves around training methods, and it only got to that point because no one was willing to give an answer that made sense.
The gist of what I'm getting from traditional stylists is that they're forced to crosstrain into a submission grappling style because submission grappling has an advantage over everything else in a ringed environment. However no one is really willing to explain what that advantage is exactly.
Saying that the canvas is level, or that there's walls in a cage, or that you can't hit someone in the balls really doesn't hold much weight. All of those factors could also exist "in the streets".
No.
If you want to do well in MMA, whatever your style, you have to train for it. That will mean practicing rounds, practicing dealing with people who want to wrestle, people who want to kick box, people who can smoothly transition from both. It means learning the rules, and how to use the rules to do well. It means doing the conditioning work for several 3 or 5 minute rounds, and learning to use that time to score points as well as seek a knockout or a submission. So some of the
training will look the same -- but that doesn't mean you have to do it in a MMA gym, or by learning BJJ, Muay Thai, or whatever. There are different ways to do that, but you'll have to do some actual application work, do the rounds, etc., and not rely on line drills, Japanese style 2 person kata, or solo kata alone. (FYI... those aren't enough for self defense training, either, generally.)
But that training will not necessarily prepare you for real violence. Nobody gets ambushed in an MMA match. Rory Miller summarized real violence the best I've seen: Violence happens closer, faster, harder, and more surprisingly than people expect. (The exact quote is in my signature.) You don't fight someone two or three times your size, with vastly superior experience. Your adversary doesn't stop when you tap, and there's no ref to step in and rescue either fighter. There is one aspect of real violence that MMA training will prepare you for, though -- and that's actually being hit, and dealing with that. Of course, physical conditioning never hurts.
Then there are different categories of violence... Broadly -- social and asocial. MMA is really social violence, only a little different from a Monkey Dance bar fight. Asocial violence is a predator/prey relationship; it's about acquiring stuff you want/need, or meeting a goal or objective. When I use force at work -- it's asocial. My job is to make the arrest -- using only and all the force reasonably necessary to do that.
So... why don't TMAs do well in MMA? Maybe it's not what they're training for. Maybe you just don't recognize them when they're there. Some arts don't contain much for a ground fight; they might well have to look for something else to complement it if they want to do MA. Just like a guy from a grappling or wrestling background might need to find a source for learning effective striking.