Or because they are polytheists?
I've followed the "Mormons are not Christians, but they call themselves Christians" issue and related it back to America's Founding, for some time. (If you have to believe in the Trinity to be a Christian, our 2nd and 3rd Presidents militantly rejected that doctrine, even though they called themselves Christians, and the evidence that the first and fourth Presidents were Trinitarians sorely lacks.)
So as Mitt Romney again throws his hat in the Presidential race, this theme will occur here again.
This time it's a hard core Roman Catholic, as opposed to a conservative evangelical who normally makes this claim, of the problem with a Mormon who claims to be Christian running for President:
"Note that I’m not in principle opposed to voting for polytheists. I could see, for example, voting for a pro-life Hindu over a pro-abortion monotheist. But a Hindu does not claim to be a Christian and thus does not risk confusing people about the core doctrine of Christianity the way Mormonism does,"...