The question as written makes an incorrect supposition. While the Islamic Empires centered in the Middle East (often called the Caliphates) did not extend over the Sahara because their supply lines would be overtaxed in crossing the desert, the religion continued to expand over that distance through trade and evangelism. A number of Sahel Kingdoms, such as Ghana, Mali, and the Songhai converted to Islam or were founded as Islamic States. On the Indian Ocean coastline, because of longstanding trade between Arabia and these places, Islam spread to the Horn of Africa and the Kenyan/Tanzanian coastline, with the several Somali sultanates (Ajuran, Warsangali, etc.) and with the Sultanate of Zanzibar off the Tanzanian coastline.