Because Christianity is an exclusive religion and anyone else is in error and is going to hell. They dislike Pagans in particular because we were the main competition for many centuries, not to mention our failure to recognize the authority of the One True Faith. Some Christians do not repudiate us though they disagree with our theology, but they seem to be in the minority. I'm not speaking first hand as a Christian obviously, but I think I'm on solid ground here. Any Christians feel free to correct me...