I think the combo of wrestling and boxing is just as successful as BJJ and Muay Thai. Machida is a very interesting fighter because he applies some Karate concepts to MMA that no other fighter has done. GSP, Liddell have karate backgrounds but they do not incorporate karate into their game. I think as it stands right now MMA is a mixture of 4 disciplines; wrestling, BJJ, MT and boxing. But there are obviously some exceptions.
Sports like boxing and wrestling have a set of rules. These rules limit the amount of techniques. Fewer techniques require participants with better athleticism to rise to the top. The type of athlete better suited to compete varies with the type of competition. For example, strength seems important in wrestling. MMA has many more techniques at it's participants' disposal than the other combat sports. This puts a lesser importance on the athleticism of mma fighters. I am not saying mma fighters are not great athletes. GSP and Anderson Silva are among the finest athletic talents in the world. The OP wanted to know about MMA versus other disciplines. A top mma contender may not be strong enough to be a top notch wrestler, may be to slow to be a top contender for a boxing title, may lack the flexibility to excel in submission grappling; and still be in contention for an mma title.
Anderson Silva and Mirko Cro Cop are both top class strikers. Cung Le is a world class striker. Wanderlei and Arlovski are both world class strikers. Also, being a world class MMA striker is just... different. It's like how a indoor volleyball player can still be really good at beach volleyball, but he won't necessarily be able to compete at the upper upper echelon levels of beach volleyball. Most pure striking disciplines have very different rulesets, not only from MMA, but from other striking disciplines. Because of this, different strategies are employed and perfected. The same way that you can see things in pure grappling competitions that would just be foolish to attempt in mma (a lot of sacrifice judo throws come to mind), the same holds true for pure striking arts. In boxing, for instance, Bernard Hopkins has perfected a style where he throws two good punches and clinches (he doesn't do it ALL the time, but he's great at it when he does it.) This is an example of a striking strategy ill suited to MMA, so it's not seen much (more by greco-roman guys). Floyd Mayweather has beautiful upper body movement that would leave him vulnerable to knees. In kickboxing there is no fear of GnP, so straight and sidekicks don't leave a fighter nearly as vulnerable. It's not that there are no worldclass strikers in MMA, it's more that MMA striking is so different than a pure striking disciplines striking that it can APPEAR that it's not world class. Of course, not all MMA fighters are world class fighters either, most probably aren't.
No there aren't that many world class strikers in mma because most mma fighters don't come from striking backrounds. Chuck Lidell wouldn't even win an amatuer boxing tournament because his technique is so bad.
