Many within the evangelical movement and Catholicism don't regard Mormons as "Christians." That's their opinion, they're welcome to it, and I could care less.
But imagine if those groups ever decided to vote as a bloc to legally ban Mormons from using the word.
Then imagine that the majority rationalized their action by saying, "We're not bigots and didn't do this because we hate you, and it's bigoted and intolerant on your part to suggest that was our motive. We love you and tolerate Mormonism, we just don't think you deserve the special right to call yourselves 'Christian,' since the Bible's teaching is very clear on the topic.
You need to recognize that Mormonism is a recent phenomenon, and you can't just expect everyone to redefine their traditional understanding of Christianity to accommodate your choice. Your use of the term 'Christian' to define yourselves is an assault on our Christianity, and we shouldn't be forced to accommodate you as fellow Christians.
And anyway, 'Christianity' is just a word. Can't you choose another one?"
I doubt that the Mormons would be quite as sanguine as they expect the equality allies to be right now.