Islam is one wacked out religion, that’s why!
How did it spread? By the sword, all of N. Africa was mostly Christian with some pockets of paganism. The followers of Islam, just like today’s fundamentalists, saw their religion as the superior faith and readily used murder and terror to “convert”.
Not that Christianity did any better when it spread through the America’s native population. The difference, however, is that these so called Christians were going against fundamental principles of Christianity, while Islam is neatly made in that murder of “non-believers” is perfectly fine