In case they want to know, it's because, to those NOT of that particular strain, such statements come across as condescendingly arrogant at best and bigoted at worst.
They say that "Everyone has a right to practice what they wish, but this is still a Christian Nation..." not even realizing how much this sounds like "We all are equal, little dears, but Christians are just a bit MORE equal...". It's a blatant attempt to assert the superiority of their own belief systems over all others.
Take the phrase "Christian Nation". Replace "Christian" with "White". Even though America is a white-majority nation, ya think that the minorities might have a wee bit of trouble with that? That they might see a WHOLE OTHER, more sinister meaning, no matter how softly their heads are patted and how gently they are cooed to about how "You're still equal, little dears...".?
"More equal" ... "Separate but equal"? Hmmm, that sounds familiar, from an earlier age when one group of people believed they were superior to the rest under God, and enforced that belief through law.
It's a blatant attempt to assert the superiority of their own belief systems over all others.
Of course it is because they believe it is superior. I can't begrudge them their belief, because why would they remain Christian if they didn't think it was superior? My problem comes when them move from personal belief in moral superiority (simple condescension can be ignored) into trying to enforce that belief on others (Ashcroft's thugs are a little more difficult to ignore).
Of course, they wouldn't have any political ends in declaring the Christian basis for America, now would they?