Great question and a bigger one than I'm qualified to answer but I'd say it had a lot to do with the slow decline of the Roman Empire and that Islam is literally the ideology of the bandit. It's so perfect for conquest. It's an evolutionary strategy for violent conquerors and it excelled at it especially when the world had not yet begun to take it seriously.

Bill Warner might rub some the wrong way but he's done some great research on this and is worth checking out. Maybe the Youtuber 'Real Crusades History' has videos on it as well but I'm not really sure.