These seem like pretty easy questions to me, so I'll offer a Christian perspective.
Although a majority of Christians today hold certain unbiblical beliefs that undermine their faith (like evolution), they do fortunately maintain the existence of only one true God and Creator of the universe. Thus, it's only reasonable that America's ~65% of professing Christians would hold prejudice against polytheism's multi-god beliefs. That consistency is only rational.
The 3-5% of America's professing Atheists, by adhering to Humanism or Materialism as a religion in itself, would also be consistent in showing prejudice against polytheism. After all, if one doesn't believe in the supernatural, negating the possibility of one god, then certainly there's no room to be considering the claims of many gods.
That leaves a sizable number of Agnostics and mixed belief people that you pose as prejudiced against polytheism. One possible explanation for this behavior could be given by referring to the Occam's Razor principle. If people are unknowing, uncaring, or uncertain about their views of a supernatural presence in the universe, it's simply easier and seemingly more likely to believe in one entity versus many. In other words, if I'm not convinced of one true Creator of everything, why would I embrace something even more complex and harder to prove? I don't think these people have reason to hate or oppose polytheism, but a mild prejudice against it seems rationally justified.
In my view, the burden of proof of any religious belief system is on the faiths' founder(s) and underlying scriptures. Here, the Christian's Bible excels above all others. For example, while the Hindu Vedas may have some historical accuracy like the Bible, and maybe even some similar manuscript authority, they don't have specific, verified, fulfilled prophesy. This alone distinguishes Christianity and the Bible from all of the world's other religions and scriptures.
Finally, presence of a supernatural creator can be shown with a preponderance of scientific evidence that we now have. Correct me if I'm wrong, but I don't think there's much if any scientific evidence supporting the notion of multiple creators. Thus, a prejudice against polytheism is justified from both a Biblical and scientific view point. Maybe that's why it's not readily accepted by the vast majority of Americans.