because they dont want their traditional style to be "exposed" for not working or what not
not to bash TMA's, and not that there arent good things to take from them, or that a TKD guy cant beat a MT guy, so on and so forth, blah blah blah.
Generally speaking, the more "effective" martial arts are what is predominantly used in MMA, due to their effectiveness. We all know what those are.
This answer is actually way off based on my experience but is a popular misconception.
As some have already mentioned most people who train have limited time and committment. They scarcely put in enough training to get very good at their own system let alone mixing it up with others. If they do get good, many get addicted to perfecting their art and competing against others of the same style at the highest level, where the depths and subtleties of the art come.
More commonly they may then want to be able to use the art for self defence application which is quite different. This may sometimes include a bit of training style vs style, but will more often include training to beat an untrained larger attacker, how to fight vs multiple opponents or against weapons which again is a commitment of time. Martial arts is a big world, and MMA is only one part of it.
From my experience only a few want to specifically train style vs style. If they want to do that they might try their hand at mma, but then they have to do again a larger time investment in training different skill sets, as well as having a deterioration in their primary style to adapt it to mma.
Then there is the issue of different rule sets and agreeing what to use. If you train style vs style it can very easily deteriorate into some form of mma anyway.
More style vs style cross sparring and training can be a good thing and force the individual to adapt his techniques to work and find out their limitations. But there will always be those who find fault with it or find the particular way you train or spar limits their skill set so it is not always so easy to make it work.
As to the point about "we all know what styles work", again this attitude is what stifles mma evolution. Dont forget in the early days of the UFC 'everyone knew' that boxing and striking basically 'didnt work' because you just had to clinch them and take it to the ground, and they were done. Then strikers evolved.
We 'know' currently what is most commonly trained in US mma and works most often. In Russia it is different with Sambo and Judo being much more prominant than in the US and muay thai less, and before Machida 'everyone knew' that a TMA like Karate 'doesnt work' as well so it is still being seen but obviously the styles that spar the most and train realistically will produce the most fighters capeable of transitioning to mma.