Because of the Arabian wars on countries like Egypt, Lebanon, Syria, Iraq.. etc. Back then most countries there were either Christian, Jews, or pagans that worship false and numerous gods, like Saudi Arabia, they had many gods, one was Allah, the god of the moon (that's why their symbol is a crescent moon)
When Mohammed said that an angel appeared to him, he was actually possessed by a demon, and he gathered a group of followers who basically like money and sex, and assembled a army to make war on unbelievers of Allah, the infidels, those were anyone who does not believe in Allah and mohammed, when he entered countries like Egypt (which was almost 100% Christian), he'd either convert them, kill them, or make them pay taxes if they wanted to live.
If it wasn't for this false religion, the Middle East would still be Christian until this day, there are many people living there that are STILL Christian, and I'm one of them... my ancestors paid taxes so that they keep their faith, and lives.
Hope I answered your question, I tried my best.