Why did most slaves come from West Africa, but not East Africa?

If you're referring to the slaves who were sent to the Americas, the answer is simple: West Africa and Western Central Africa are relatively closer to the Americas as compared to where the East Africa is - so it purely has to do with geographical locations.

But if you're referring to the African slaves as a whole, there were also North and East African slave trades, not ONLY the Atlantic slavery trade from West Africa and Western Central Africa!

It's sad to learn that when people talk about the African slave trade, they only talk about the Atlantic slave trade (which actually constituted about 55% only of the whole African slave trade). In addition to it, there was North African slave trade (Arab slave trade) and East African slave trade (Arab and Portuguese slave trades). The East African slave trade used almost all the seas and oceans of the world and even a river, to transport slaves – the Nile River (in vessels of papyrus), Indian Ocean, Red Sea, Arabian Sea, Persian Gulf, Atlantic Ocean and as far as the Pacific Ocean. The East African slaves were taken from “Beyond the Rivers of Ethiopia” countries - which are the South-of-Saharan African countries (Isaiah 18:1–2, 7 and Zephaniah 3:10–13),……and were sent to the present day Middle East or Levant, Central Asia, Far East (Japan, etc.) and even to the Americas, as those taken from the west and central Africa.

Have you ever heard of the Black Samurai Yasuke of Japan? He was among those slaves or precisely servants, from the tribe of Makua, which straddles between present-day Tanzania and Mozambique, who was sent to the Far East from this part of southeastern Africa, possibly from the present day Tanzania, through the City of Kilwa (which was one of the major slave ports then). This was before the Scramble for Africa, when this part of the present-day Tanzania was a Portuguese settlements (not colony, that came 300 years later for what became Mozambique).

All in all, the East African slave trade started in the 18th century AD, as detailed in the link below and as narrated at the main East African slave hubs — Bagamoyo, Zanzibar and Kilwa, all in present-day Tanzania. I, myself, originally from there, so I know this history by heart. People from my own tribe were slavery victims.