krav maga utilises techniques from a variety of martial art and self defence styles and as a result can be closely compared to MMA. krav maga however does contain techniques that are deemed illegal for use in mma such as throat gouges groin shots eye pokes etc and therefore the system would have to be adapted to accomodate the rules of mma. this also applies to other styles to. i think the fighter ryo machida has a strong karate background and uses techniques in his style, additionally his stance is very similair to that of karate but herein lies the practicality of martial arts, they would all need to be edited and 'tweaked' to suit the conditions of cage fighting. even boxers have to alter their stance. additionally certain techniques like some found in aikido are used to simply redirect an opponents attack and often result in very little to no damage, which would not be as desired as a wrestling slam or muay thai kick.
Those arts are for lifelong commitments. Traditional practitioners seek to better themselves, their lives, and their attitude. MMA is a sport. It uses arts that are best for that sport. MMA is for money, fame, and because its a great entertaining sport. But the martial path brings you closer to peace. With yourself, your life, and your enemy. The Japanese call it Budo, way of the warrior. MANY will say that these arts are not in UFC because they "do not work" they are right, MMA is for sport and entertainment. Those arts are a lifestyle. An art is only as good as a user. You ask if they do not work, that depends on your definition of work. Do they look entertaining, no, can they be learned quickly, no, do they follow the rules of MMA, no. If by not work you mean not be able to defend yourself with, you are sorely mistaking. These arts have been around longer, and some have been used in actual wars. MMA evolved over a period of trial and error, seeing what combination of arts make a good sport. If you do not see it in MMA that does not mean it is not effective. If I beat you with a stick lets say, thats not in MMA but it still works.