There's a fairly common standard among mainstream Christian groups as to what constitutes the boundary of Christianity formed by adherence to certain understandings of foundational doctrines (basically encapsulated into the common Nicene and Apostle's Creed's.)

This is generally held to exclude (as I recall), for different (though overlapping) reasons, Mormons, Seventh Day Adventists, Jehovah's Witnesses, and Unitarians, among others groups that describe themselves as Christian. (In contexts where the definition in question is being used, these and similarly situated groups may be referred to as "pseudo-Christian cults".)

This definition isn't consistently important outside of academic theological contexts within the body of mainstream Christianity, though, as lots of regular believers use either a much broader definition in daily conversation, or a narrower one that amounts to agreement with the particular religious beliefs of the individual in question.

Particularly, in US political contexts, political groups organized around politically conservative policies justified by appeals to Christianity have very often sought to appeal across traditional theological boundaries to reach politically-significant populations, and this is absolutely the case with trying to draw in LDS members; so, in terms of conservative political Christianity, Mormons are usually accepted, even if there is an academic theological standard which holds them as distinct from Christianity.

There was a fair bit of established orthodox doctrine laid down between 100-400 CE. Christian groups that follow this are generally considered Christian by each by each other. Mormonism deviates significantly from orthodox doctrine, so it is considered a heretical sect by Christians.

For analogy, comparing Mormonism to Christianity is sort of like comparing Chrome to Konqueror.