Hi
This might seem condescending and narrow-minded to people of other faiths, for which my apologies in advance...
But in my opinion polytheism failed for one very simple reason: it was man made and thus inevitably flawed. The same reason why all other religions will inevitably fail, except for Islam since it isn't man-made.

Oh and also what Aamirsaab said, omnipotence only works if it's exclusive. If two Gods are omnipotent one could limit the other, bringing a flaw in it's omnipotence.