EPO became available to athletes in the late 80s, which is one reason Dr. Rosa ran off to Kenya. He figured he could experiment without interference and he was right. Hermens followed suit, as did others. Even though testing has gotten better (where are all those sub 12:50 5k runners lol) the E. Africans and others are let dope off season / out of competition. It isn't only the E. Africans, as certain third world countries are allowed to do whatever out of season (Cuba for example) and nothing is said or done.
When you are not being bothered by tests you can find talented athletes and from them you find the best responders to drugs. Sure, some idiots lie and tell us that EPO "doesn't work" on Kenyans for example, when in fact Kenyans and their handlers have fouled up and been caught. Sure, politics has saved the day (Lagat for example, or the Kenyan fiasco in Rio with 'stand ins' lining up to take drug tests and most of the story being covered up) but some Kenyans and others have been caught using a drug that doesn't work for them. It's a side splitter.
Europeans became intimidated by the numbers of Africans (inflated by rampant drug use) and the ridiculous times (Geb running under 12:40 lol). The Americans have shown that non-Africans can in fact run well, and that's with the sport not being all that popular in America. The huge increases in the money that has been poured into soccer since the late 80s hasn't exactly helped middle and long distance running in Europe either.
The E. Africans have a huge number of talented people but the numbers we've seen since '87 (never mind most of the top performances) have been inflated and exaggerated by drug use. If the sport were totally cleaned up (and political forces do NOT want it to be cleaned up) and Europeans began competing in the numbers they did up through the mid 80s, the Africans would still have the highest percentage of top performers, but they wouldn't be nearly so dominant.