Why do UFC or MMA fighters only learn standard skills?

People think its impractical to learn anything else. At one time you were laughed at for practicing Karate. GSP and Bas Rotten were the only famous guys who practiced it. Now that Machida has made it more famous people consider it to be practical. Taekwondo is another martial art that is belittled but guys like Anderson Silva, Ben Henderson and Anthony Pettis have black belts and one day more taekwondo practitioners will come out and people will take taekwondo seriously. The same thing can be said about Kung fu practitioners and Silicon practitioners. It takes guys that can adapt a style to MMA in order for people to consider it to be "practical. Before people thought those Crane kicks and front kicks used by Machida and Silva were impractical but they proved people wrong and one day other techniques will make the other martial arts "practical".