Its largest organization, the UFC, has a wide variety of races and countries represented among its professional athletes. Even the best in this sport are extremely diverse. This isn't something you see in many sports. Football and Basketball are dominated by Americans, baseball has a large latin American population but doesn't have near the European or African representation. Hockey has a lot of candians and Europeans in addition to Americans but glaringly lacking in non white dudes. Soccer is very worldwide, but the various leagues stick to their area aside from world cup every 4 years. They're not going against each other with as much normalcy as the UFC.
(Yan was born in Russia for comparison)
Then not just the athletes, but how the sport is conducted. Every other sport, women are in their own organization. WNBA. etc. Women use all the same facilities as the men, no smaller arena for the smaller crowds for them. No other sports will show a men's competition, women's, back to the men's, and then even have the top showing be a women's event.
As far as rules, other than wearing tops, there's no rule differences between the men and women. Hockey, there's no checking for the women. Golf, the women tee up closer. But it's not like in MMA women are seen as fragile and so can't handle the knees or elbows
As barbaric as the sport is, I think it has to be recognized as the most diverse professional across both gender and racial lines.
A third pointer on your post. Have you forgotten about the European cups? Europe's top teams from every country play eachother every year to crown a European champion, there is also the European Championships for national teams. So no, they don't just play in each other's leagues.
Also consider the domestic cups where amateur teams can face off and potentially win against the giants. Do you see in MMA that an amateur gets a shot at the top 10 in UFC?