I don't get why people always tout this as if it's an accomplishment. It's the fastest growing because it's the newest sport, how is that something to brag about? If you want to talk about size it's still one of the smallest sports in the world...
MMA isn't the newest sport. There have probably been a thousand sports which have never took off since then. By your standards, we should watch out for XARM to be on PPV any day now.
Watch UFC 1 and watch UFC 153. Boxing has been around for what? A minimum of 100 years? MMA has been around for like 20, and the UFC is already beating the majority of world title fights in boxing.