In short:
The reason Mormonism is not Christian is because it denies one or more of the essential doctrines of Christianity. Of the essential doctrines (that there is only one God, Jesus is God in flesh, forgiveness of sins is by grace alone, and Jesus rose from the dead physically, the gospel being the death, burial, and resurrection of Jesus), Mormonism denies three of them: how many gods there are, the person of Jesus, and His work of salvation.
I had many mormon friends in high school and they used to tell me about how God is only one of countless gods, that he used to be a man on another planet, that he became a god by following the laws of that god on that world, and that he brought one of his wives to this world with whom he produces spirit children who then inhabit human bodies at birth. Um?
Spirit Children??? I rest my case.