Thirdly, I wrote "most", not "any". If few mass killings were recognised as genocide, that changes nothing: every rule has exceptions. And, lastly, Mengistu became a "Communist" by pure coincidence: during the conflict between Ethiopia and Somalia both states tried to get a support from the USSR. The Soviet leadership, for unclear reason decided to support Ethiopia - and this barbaric regime suddenly became Communist whereas another one became anti-Communist. In actuality, I doubt any serious studies can confirm that that sudden conversion of Mengistu into Communism caused any increase of mass killings (and, similarly, I doubt Somalia without Communists immediately became a democratic and humanistic state: as you see, this state is even more barbaric that Ethiopia).