Wrestlers have long had and continue to find success in mixed martial arts. The top ten of every weight class is littered with former collegiate and Olympic wrestlers (27 of the current 70, based on last month's SBN/USA Today rankings), along with other fighters whose base style can be classified as wrestling but who didn't achieve that level of amateur success (GSP, Melvin Guillard, Josh Barnett, and Eddie Alvarez, for example). There are a variety of reasons for this success, not the least of which is the simple effectiveness of the art itself: wrestlers can dictate where the fight takes place due to both their takedowns and their top-notch takedown defense. It's difficult to overstate the importance of the wrestling skill set, but overemphasizing that skill set blinds us to the ancillary factors that make wrestlers such outstanding fighters. Wrestlers find success in MMA not simply because of their ability to wrestle, but also because the structures of amateur wrestling as a sport incentivize particular attributes that allow them to be successful. The most significant of these attributes are the ability to cut large amounts of weight, an emphasis on a team structure for training, and most importantly, athleticism.
Cutting weight is a practice common to all combat sports. No athletes, however, are as adept at cutting large amounts of weight as amateur wrestlers. I would argue that this is a function of the frequency with which collegiate wrestlers engage in weight-cutting: take a look at the schedule for the University of Iowa's wrestling team this year. Over a four-month season (including the conference and NCAA championships), they will compete, and therefore cut weight, about once a week, and occasionally more often than that. That puts a tremendous amount of strain on the body (as Daniel Cormier's damaged kidneys attest), but this experience serves amateur wrestlers well once they make the transition to MMA. While boxers, kickboxers, and jiu-jitsu fighters also cut weight, they simply don't have the level of comfort with the process that wrestlers possess. In practical terms, this can create a significant size advantage for wrestlers on the night of the fight; if they are comfortable cutting ten, fifteen, or even twenty pounds of water weight the day of the weigh-ins, and are confident in their ability to rehydrate, then they are already a step ahead of their smaller opponents. One could cite examples ranging from Ben Henderson to Brock Lesnar. While smaller fighters routinely beat larger ones (Frankie Edgar, anyone?), when dealing with a large enough sample, the effects of a size advantage are apparent.
The second advantage that amateur wrestlers bring to the table is the emphasis on team training. The current team-dominated landscape of MMA has its roots in wrestling: Team Quest, founded by Dan Henderson, Matt Lindland, and Randy Couture, and Miletich Fighting Systems, the first two major camps based in the US, were both founded by individuals with a strong wrestling base. That pattern still holds true today: highly successful amateur wrestlers, even at the beginning of their careers, tend to join successful and comprehensive teams, where they then acquire the skills necessary to become well-rounded mixed martial artists. Of the twenty-seven top-10 fighters with a base in amateur wrestling, twenty-four of them fight out of major camps (the exceptions are Rick Story, Scott Jorgensen, and Ben Henderson, though the last is debatable). It's difficult to overstate the importance of having a good camp, especially for prospects: the first few years of a fighter's career are almost invariably spent developing the basic skills in submission grappling and striking that are necessary to excel in MMA, and a top-notch camp accelerates this process. The American Kickboxing Academy in San Jose, for example, seems to specialize in taking amateur wrestlers and rapidly teaching them submissions and striking.
The final reason for the success of amateur wrestlers in MMA is the most important, the most complex, and the most difficult to define in simple terms. Simply put, amateur wrestlers tend to be better athletes than their opponents, as measured in terms of speed, explosiveness, strength, and the power in their strikes. There are several different reasons for this, some of which have to do with the sport of wrestling itself, and others with the broader context of sports in American society.
Wrestling has substantially fewer techniques and variations on techniques than, say, Brazilian Jiu-Jitsu. In terms of training time, wrestlers practice the same basic moves over and over again, to an utterly grueling extent. They'll switch from the double leg to the single leg to the high crotch, ad nauseam. I'm not at all saying that wrestling is easy to learn, or that it's a simple sport, just that compared to the hundreds or even thousands of techniques that a BJJ black belt has trained and drilled, an amateur wrestler's training will have been much less focused on learning a wide range of techniques. Since wrestlers aren't likely to hold a substantial technical advantage over their opponents, the athletes who reach the highest levels of the sport tend to be the most athletic. It also helps that the basic techniques in wrestling incentivize athleticism in a way that jiu-jitsu (particularly in a gi) doesn't: the stronger and more explosive you are, the more likely you are to land a takedown, while holding an opponent in side control doesn't require the same brand of raw athleticism, especially if you can hold onto their clothing. Moreover, athletes aren't just born, they're made: training programs for collegiate and Olympic wrestlers reflect this emphasis on explosiveness, and they are far more likely to have spent years doing intense strength and conditioning work, especially lifting weights, than their counterparts in other combat sports. There are of course hyper-athletic jiu-jitsu fighters like Jacare Souza, but they tend to be the exception rather than the rule. I don't think anyone would call Demian Maia, Big Nog, or Roger Gracie a world-class athlete in the mould of Jon Jones, Chael Sonnen, or Josh Koscheck.
The economic incentives for wrestlers are an important variable as well. When an amateur wrestler graduates from college or finishes training for the Olympics, they have essentially reached the end of the marketability for that single athletic skill set. A tiny minority might go into professional wrestling, a la Lesnar or Kurt Angle, or even into the NFL (like Stephen Neal) but those options are essentially unavailable for the smaller weight classes. They could go into coaching, but nobody strikes it rich in amateur wrestling; even the head coaches at top universities only make around $100,000 per year. MMA is the most lucrative option available to amateur wrestlers, and they've embraced it wholeheartedly. Bubba Jenkins, the 2011 NCAA champion at 157 lbs, has already begun training at American Top Team and will make his pro debut shortly; he had already decided to go into MMA even before finishing his college career. On the other hand, why should Tyrone Spong or Gokhan Saki (probably the most athletic and explosive heavyweight kickboxers) take up MMA? There was no economic incentive do so, at least until K-1 effectively folded. Now both are thinking of making the transition to mixed martial arts, and their athletic talents alone would give them a leg up on their competition.
By the time a collegiate wrestler graduates, they are already the product of an organized system of athletic competition through middle school or junior high, then high school, and then college. At each of these stages, the system has effectively selected the most athletic prospects and then spent years developing that athleticism by means of strength and conditioning programs. Without another viable means of making money from their athletic gifts, these prospects then join top-notch camps, where they are surrounded by outstanding coaches and training partners and quickly learn the essential skills of MMA. When they get into the ring or cage for their first several fights, their ability to cut weight means that they're larger and stronger than their opponents, especially at the local or regional level. Even promising prospects with a base in other disciplines can quickly be overwhelmed by the size, strength, and speed of their opponents. It shouldn't be surprising that these fighters quickly make it to the highest levels of MMA: they possess structural advantages that simply aren't available to practitioners of other combat sports.
Of course, the ability to put your opponent on his back doesn't hurt, either.