onedreamer
Dragon
First, it has already been discussed that ancient religions were always tied to certain peoples or cultural groups: the Romans had Jupiter's pantheon, the Greeks had Zeus' pantheon and so on. For every ancient polytheistic religion, there are no historical examples of religions spreading to another culture and becoming its dominant religion.
although I like your idea about cultures, I disagree with this statement. First off, it would be like saying that "God" and "Deus" are different gods. They are the same God called with different names in different languages. There is no Jupiter or Zeus pantheon, they are exactly the same with different names. The "stories" of the Gods were also the same... Now, other polytheistic religions had Gods with different stories, but the main figures are present in any polytheistic pantheon: the God of universe, of sun, of fertility, of war, of fire, etc.
For this exact reason I think we shouldn't be talking of different polytheistic religions, it is in essence just one and that is why no ancient polytheistic religion spread to other cultures already adopting a polytheistic religion: because they were essentialy the same there would be no reason for this to happen.