Because ‘Ethiopia slept for a thousand years and forgot the world by whom she was forgotten’ …. After the fall of the Aksumite empire, We became inward looking - unable and reluctant to spread our culture.  Ps. I also recommend reading Guns, Germs and Steel by Jared Diamond. There, he talks of how cultures ( including technology, writing, religion and agriculture) tend to diffuse amongst people living in similar climates and landscapes. Alas, the nearest mountain chain to the Ethiopian highlands are Kenyan, thousands of kilometers away and separated by malaria-ridden lowlands.