Why so many champions from this small region ?
Simply, it is because of a fortuitous combination of circumstances that bring a genetic potential and a stimulating environment together; in other words, even more simply, "the right body in the right place".
The Right Body
Most of the champs are from the tribes that from time immemorial were nomadic pastoralists and had, as a matter of survival, to travel long distances with their livestock. Undoubtedly, natural selection ensured that genes conferring strength and stamina were passed on in successive generations --- the long, lean (skinny) look is a morphological manifestation. Other genes with a role in metabolising food might also be a factor.
The Right Place
This seems to be an environment were the body has to really use, from an early age, its inherited capacity to walk and run; commonly, a high altitude environment of hills and valleys. Such environments account for about 20% of North Rift and in these areas children travel many kilometers, on foot, daily, between home and school and /or between home and the shamba (cultivated fields). Often the journeys involve part of a steep escarpment; vertical distances covered may be hundreds of metres. Under these conditions the children maximise their genetic potential and develop efficient skeletal and heart muscles.
The Altitude Factor
Great muscles can only function greatly if they are supplied with enough oxygen. At higher altitude the atmospheric oxygen is less (and gets progressively less the higher we go), so inhabitants of mountains and tablelands "adapt" and produce a higher concentration of red blood cells in their blood. The ability to efficiently supply oxygen to lungs and brain is thereby maintained. This is a natural body response which all people display and can take advantage of. After only a few weeks of living at a higher elevation, a person's red blood cell 'count' will be raised. This "compensatory" effect has no real advantage until that person returns to his normal, low-altitude abode; here the extra blood cells in his system will be a real advantage, carrying oxygen to his muscles at a faster rate when required and he will be able to perform better. Sportsmen, particularly athletes, take advantage of the "altitude factor" phenomenon ---- no competitor can afford to go into a high-profile competition without having done at least a few weeks training at altitude.
Of course, the indigenous inhabitant of the North Rift highlands, where the altitude is typically somewhere between 2000m and 2700m, has no need for those few high-altitude weeks; he or she has, effectively, been training at high altitude since birth.