Islam is a warrior cult, organized under the banner of monotheism, that was able to take advantage of the achievements of older civilizations and cultures in the Middle East to advance itself. Evidently there was, in those times, some tension between the Coptic, Egyptian, and Syrian cultures and the Byzantine Greeks---and they thought the Byzantines were harsh and oppressive. The Byzantines, though, had been wiped out by plagues from Egypt or Sudan, but also had the job of defending the eastern empire from invading groups like the Huns. Which probably is the main reason they had to levy taxes in Palestine and ancient Syria.