It became popular because people were tired of Karate/TKD/Kung Fu/Krav Maga morons telling everyone their martial art can "kill in one touch" or something. So, Pride and UFC started and the entertainment was watching martial arts compete against each other in a near luta livre style. The idea of course being to find the best martial art. BJJ, Wrestling, and Muay Thai were the only survivors.
Once the global maxima was found it became relatively boring. At least to me. But lord was it fun watching "masters" of traditional martial arts get beaten up and down the ring and all the magic of their martial art going along with them.