According to experts, about 60 to 80 % of Olympic medalists are doping.
Europeans like Mo Farah have access to Doctors and other experts who help them beat the dope testers using elaborate techniques like micro-dosing. Mo Farah's coach is Albert Salazar who is known for pushing the envelope.
Kenyans are being caught in large numbers because their doping is rudimentary and incapable of fooling a tester.
And Ethiopia apparently does not even have a legit dope testing regime. Some like Genzebe Dibaba are obviously doping.