Why couldn't there be multiple gods (i.e. polytheism)? 

That’s a good question, and I'd answer it three ways.

One is Occam's razor. In the absence of any evidence for multiple independent causes, In other words, when comparing one god to multiple gods, it is simpler to say there was just one.

Two, most polytheistic religions claim that the gods themselves came from an earlier source, and are not timeless or fundamental. Even if those multiple gods did exist and created the universe then we would need to go back to a more fundamental power that we might call the real God. In that case, it reduces into a kind of monotheism.

A third way to think about this is that a single God explains the rationality and regularity of the universe better. When we look at the universe, it appears, as much as we can tell, to be rational and regular. That is, it follows physical laws, and those laws seem to be unchanging no matter where you are in the universe. What's true of physics on earth is true on the moon, and on Neptune, and so on throughout the universe. In fact, that's a basic assumption of science, that the universe is a coherent whole, and it's why scientists try to uncover theories that connect all our observations together. But if there were multiple gods truly independent from each other, why should the universe follow a single set of rules, or be rational and regular? What makes them agree? We can say it's more likely for there to be one fundamental rational mind that explains our rational universe.