In the early 1990s, three styles stood out for their effectiveness in MMA competition: Amateur wrestling, Brazilian Jiu-Jitsu and Shoot wrestling. This may be attributable in part to the grappling emphasis of the aforementioned styles, which, perhaps due to the scarcity of mixed martial arts competitions prior to the early 90s, had been neglected by most practitioners of striking-based arts.
Even though fighters combining amateur wrestling and striking dominated the standing portion of an MMA fight, Brazilian Jiu-Jitsu stylists had a distinct advantage on the ground. Those unfamiliar with Brazilian Jiu-Jitsu proved to be unprepared to deal with its submission techniques. Shoot wrestling practitioners offered a balance of wrestling ability and catch wrestling based submissions resulting in a generally well rounded set of skills. The shoot wrestlers were especially successful in Japan, where the martial art initially dominated other arts.
As MMA competitions became more and more common place, those with a base in striking became more competitive as they began to acquaint themselves with takedowns and submission holds, leading to some notable upsets against the dominant grapplers. Subsequently those from the various grappling styles learned from each other's strengths and shortcomings and added striking techniques to their arsenal. This overall development of increased cross-training resulted in the MMA fighters becoming increasingly multi-dimensional in their skills.
I've only been in two fights in my life, but I won both of them using a single devestating move: a punch to the stomach. That may not sound like much, but I'm undefeated (and retired).