MMA is already an american thing like football, baseball and basketball.
There will be some champions from othe regions, but the core business will be the USA. This kinda make other places not to be encouraged to practice these american sports because of the american dominance. Just like almost nobody else in the world practices american football. Also, the US is a huge country. You can have healthy core centers for MMA in many places (Cali, NY, Texas...)
You can train boxing and have elite training camps in Mexico, Russia, UK, Japan, Cuba and some other places. You just need a pair of gloves and some elite sparring partners and coaches that you can find in many countries It helps also that there is not a clear superpower for the sport. You can become an elite boxer or soccer player coming from many places in the world. The South America - Europe fight for dominance in soccer makes it a lot more popular and decentralized thing. People is looking for real globalized sports. FIFA world cup is an event hold in all the continents of the world, and they make tons of money from it in any country is hosted. They think globally because they profit globally. The UFC is an American company, they think locally. Their european and asian cards are usually lackluster, full with c-level athletes. Their cashcows usually fight only in the US. Conor talked a lot about fighting in Dublin, but this is the one thing the UFC has not allowed him to do. They need them murican money. This refrains the sport of MMA to become a really global thing.
Besides, training MMA is a complex and expensive thing. You need boxing coach, wrestling coach, BJJ coach, Muay Thai coach etc. The USA is the only place in the world with a complete infrastructure for this. Even top brazilian talents go to train and live in the USA for this reason. The best russians also go to train in the US. Whitaker, Conor and Adesanya are the exceptions.
MMA faded in China before it became popular in there. The UFC is never going to put their best cards in chinese territory. Japan was once the epicenter of global MMA less than 20 years ago and now is almost dead in here (yes, I happen to be in Japan right now... A coincidence, going back to Colombia in a couple of weeks) And is because japanese were used to have the best MMA fighters in the world fighting in japanese soil. The US dominance made MMA popularity in Japan fade away. And this may happen in Brazil as well.
On the other hand, Kickboxing is obviously a poorer sport than MMA, but you have different talent pools and epicenters in many places. It has many requirements for becoming a global thing. You have elite talent training in Thailand, Brazil, UK, Holland, France, Russia, Ukraine, China, Japan and in those ex-soviet nations. You have events with top talents happening in Europe, China, Japan, Thailand and even the US. Is not enough popular in the USA so allows the developing of the sport in many other places... They just a new Badr Hari, or Conor abandoning MMA for Kickboxing, and the sport may explode globally. Here in Japan Kickboxing is already more popular than MMA right now. Takeru and Tenshin are the Brock and Ronda here. They have top talents fighting here the whole time in Rise and K-1. And the same happens with Glory-Enfusion events in Europe and Kunlun - WLF - GoH in China. Adesanya was a fairly popular fighter in China before deciding to do MMA and abandon Kickboxing for good.