Wrestling is dominant because MMA is the only real option for wrestlers who aren't good enough for international competition, or simply don't want to compete internationally anymore. There's not alot of money in being an amateur wrestler either, so MMA can be quite attractive to wrestlers. The same is true for BJJ, but there aren't nearly as many BJJ guys as there are wrestlers, especially in the states.
Stirkers, however, have lots of alternatives than can be much more lucrative than MMA, so that's why you don't see nearly as many dominant strikers. Even the guys who have done well in the past like Chuck and Wand with their striking were never really great strikers who would do well in K-1 or boxing. Right now, Anderson and Overeem are really showing what world class strikers can do in MMA, but there aren't many guys on that level.
Until MMA is more of a lucrative career, we're going to see alot of wrestlers in an american company like the UFC.