I won’t pretend I know the exact answer to this, but I just took a class on Islam and I will try to recite what I know.
Islam is growing rapidly, mostly in Africa, Asia and the Middle East to the best of my knowledge (as well as parts of Europe). Some of the beliefs and practices of Islam are similar to ones that the tribes in Africa had. Muslims are known for trade and that is just one way that they can spread their religion. Another is through conquest which is how Islam was spread to many parts of the world.
Part of the situation in Africa is that the colonialists were all Christian and seen as hostile. Well, this, to say the least, discourages people in that region from wanting to even think about converting to Christianity because of the way that the colonialists acted. The Muslims traded a lot with the people in North Africa and had a more peaceful means of being able to covert people since they were not there to conquer (as were the white, European Christians).
That’s about all I know about the spread of Islam (or at least what I know that pertains to this topic).
I do not know what converts to Islam feel or why they convert. I have not taken any surveys. It may be that the situation is the same in many other countries where white, Christian people came in to colonize their nation and because of that it gave them a bad impression of Christianity.
It also might be all the weird versions of Christians that are out there trying to give everyone a Bible thinking that it will change everyone’s life (not that I find anything wrong with the Bible of course, but I just get fed up with the people that think we need to send Bibles to China or something without actually taking it in context of the rest of Christianity).