First of all Kazakhstan is part of Central Asia. Secondly they are a war torn region, the Persians, Turks, Mongols and Soviets have torn through Central Asia over the last millennia or so. Beyond this, Soviet influence is huge on combat sports similar to what it did to Cuba in Judo, Wrestling and Boxing. Wrestling is a very popular sport in Tajikistan, Kazakhstan, Kyrgyzstan and Uzbekistan, as is boxing, Judo, Tae Kwon Do and even Sambo is quite popular relative to the rest of the world. It's not high altitude or special bloodlines, it's really simple. Plus, there aren't major athletic outlets for people in Central Asia to take over MMA, at least not in the same way like there is in the West with Soccer, Rugby, US Football, Basketball, Hockey, Baseball, Tennis etc. When the monthly average income in Kazakhstan is 700 USD, those 15k/15k contracts look fucking amazing.
Plus in a lot of these regions, fight culture is just strong and more popular. For instance people don't know but Khabib did Pankration and Hand to hand combat tournaments growing up along with wrestling and Sambo before focusing souly on MMA. Similarly Islam started in Tae Kwon Do, then Sanda, then Wrestling and then Sambo all between the ages of like 8 and 14 before starting to train with Abdulmanap full time. A lot of these Central Asians have similar backgrounds, Karate, hand to hand combat, Sambo, Unifight, amateur MMA tournaments, boxing, Tae Kwon Do, Judo etc etc. It's just more popular and taken more seriously and at younger ages in general.