Actually, the Middle East used to be a region of enormous religious diversity. It is now less diverse than ever, and is losing diversity.
For the last 1400 years Muslims have been dominant, but by no means always in the majority.
Islam itself used to be far more diverse than at present. Saudi money and fundamentalism have cut way down on local variability.
It’s amazing how money has the mystical power to convert even the most devout Imam to the Saudi way of thinking.