The civilization was large enough to implement many different worldviews. Some of them were weaker and died out. Some of them were stronger and they survived. A church of a single creator is unified and as such is stronger than a bunch of temples of different Gods, each with their own mini-religion.
The paradox is that such a powerful idea came from one of the weaker nations in that world. But it was more than an idea of course. The ancient Egyptians came up with the idea of a single god, but Aton was not a creator. Nor was the God of Plato. What made the idea so powerful was that this creator was also a person like the several gods of the pagans. It basically swallowed up the many gods, reducing them to lesser beings, such as angels and demons.