The vast majority of religions historically have been polytheistic (or atheistic).
Islam and Christianity being monotheistic was probably relevant to their expansion because they proposed a total replacement of the prevailing polytheistic belief system wherever they spread. It seems usually when polytheistic religions collide, their gods tend to merge into a syncretism.
Another way to think about it is that it's not their expansion that's remarkable. It's the fact that they preserved the original system so well, despite expansion across literally multiple continents and cultures. For this it helps to have one prophet, one book, one God (, one Pope / one Mecca).
Islam and Christianity being monotheistic was probably relevant to their expansion because they proposed a total replacement of the prevailing polytheistic belief system wherever they spread. It seems usually when polytheistic religions collide, their gods tend to merge into a syncretism.
Another way to think about it is that it's not their expansion that's remarkable. It's the fact that they preserved the original system so well, despite expansion across literally multiple continents and cultures. For this it helps to have one prophet, one book, one God (, one Pope / one Mecca).