When did this come down to theology? But since we are here, Christianity is based upon Judaism. Christianity does not rewrite Judaism, but rather postulates that the Kingdom of God as noted in Judaism is in essence not an earthly or physical kingdom, but rather a spiritual one. That spiritual kingdom was created with the Messianic prophecy of Judaism being fulfilled through Jesus Christ.
There are plenty of Christian denominations with various interpretations, but for foundation must be consistent for them to be Christians. I personally feel Catholics err too much on legalism and creating hierarchies with Saints, Popes, and Mother Mary. However, Catholics are Christians in they do not completely rewrite Judaism AND they hold the belief in the divinity of Christ.
Mormons invoke the name of Christ, however their fundamental beliefs are based upon a complete rewriting of Judaism and conjecture without any proof. And before some purists talk about lack of proof in in the Bible I am referring to the fact that at least Judaism and the Old Testament refer to kingdoms we know exist such as Babylon and Egypt. Whereas the Book of Mormon has conjecture about the true Isreal being in the Americas and that white people were there before Europeans ever discovered the land. Also Christ's story in Mormonism goes beyond the crucifixion which is unlike any other sect of Christianity. All that being said, Mormonism has actually been mostly positive for its followers and society. Nothing is perfect, but the teachings make for good neighbors and communities. So while saying that Mormons are not Christians in the same way Baptists, Methodists, Lutherans, Catholics, etc... are is not disparaging or condemning them as evil.
However, Mormonism is like Islam in that a singular person contradicts the basics of the 39 books of Judaism/Old Testament as well as the 27 of the New Testament which were written across multiple generations and by multiple authors. No matter what your stance on Religion is, there is a difference between a set of beliefs whose fundamentals are based upon multiple accounts across multiple generations and a set of beliefs whose fundamentals are based upon a singular person creating them over a blip of history.