When I say Eastern Africa, I should probably confirm I am just talking about the area under Egypt to Ethiopia along the coast. After playing CK2 I realised this whole region is essentially Christian so it also makes me wonder why they didn't join the crusades at all.
[Edit] I'm aware they weren't of the same denomination or anything as Catholics or even Orthodox
Not to discourage any further answers but you might be interested in this answer to What did European Christians in the Middle Ages think of Ethiopia, and vice-versa? by /u/Compieuter.