I heard a radio story about things like "Kenyans are fast" and basically, there is a culture in Kenya at the moment that celebrates runners. So, more Kenyans try and they try harder at running than they would minus that cultural emphasis on being a good runner. More Kenyans training and being involved in that established championship running culture finds the best amongst the best and prepares them to win. Distance running is a practice in enduring pain. If the expectation is that you can endure the pain it turns out you're more likely to endure the pain.
Does Kenya have the worlds best distance running establishment at the moment? Yes. Are Kenyans genetically good runners? Maybe, but not so much that it accounts for their competitive success all by itself.