To clarify, I am wondering about its astonishingly quick soread, at least as compared to other religions, and its wide reach, from the sultanates of West Africa to those of Indonesia.
Here are some other questions on the same topic that might give you some answers:
Long story short, Islam didn't spread as quickly as you might think. Areas in the Middle East, North Africa and parts of Europe fell under Islamic rule with the rapid pace of the Islamic conquests, but Islam itself was not the predominant religion in those regions until much later, after centuries of rule. In Arabia and North Africa people also experienced a culture shift over time, not just a religious one. Areas outside of the direct control of the Caliphates also converted in some cases, particularly along the trade routes of Asia, which gives us what might otherwise be perplexing examples of primarily Islamic countries today, such as Indonesia, far removed geographically from much of the Islamic World today.