Theory: If you want to fire up a bunch of hotheads to "go forth and conquer", then a superficially monotheistic religion is a hot meme.

(Actually having a Trinity, 13 different orders of angels, thousands of saints, hosts of demons, etc. is not a real problem - so long as the guys with the swords are free to skip the details.)

But as usual with history, "what about the Mongols?" The were arguably the greatest conquerors in human history. Yet famously tolerant of a wide variety of religions - https://en.wikipedia.org/wiki/Religion_in_the_Mongol_Empire