Because it deviates from Biblical Christianity in a number of irreconcilable ways. I will mention just a few here. For a more in-depth explanation of why the RLDS church is a cult, see Carol Hansen's book Reorganized Latter Day Saint Church: Is it Christian? See the section entitled Anatomy of a Cult, particularly chapter 4, Earmarks of a Cult.
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Extra-Biblical Revelation. RLDS hold as sacred Joseph Smith's Book of Mormon, Doctrine and Covenants and his Inspired Version of the Bible. Each of these books seriously conflicts with the Bible. They introduce a myriad of un-Biblical beliefs and doctrines. For an in-depth discussion of each of these books, see chapters 4-10 in Part Way to Utah: the Forgotten Mormons, and chapters 9-11 in Carol Hansen's book Reorganized Latter Day Saint Church: Is it Christian?. Our tract Malachi and Joseph Smith provides a multi-faceted example of these serious conflicts.
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A Different Jesus. Sadly, the scriptures and resultant theology of Joseph Smith introduce us to a Jesus which is very different from the Biblical Jesus. This is nothing new, it has happened right from the start of Christianity. It is precisely what Paul scolded the Corinthian church about. "But I am afraid that just as Eve was deceived by the serpent's cunning, your minds may somehow be led astray from your sincere and pure devotion to Christ. For if someone comes to you and preaches a Jesus other than the Jesus we preached, or if you receive a different spirit from the one you received, or a different gospel from the one you accepted, you put up with it easily enough" (2 Cor. 11:2-4, NIV). For an excellent discussion of the "other Jesus" of Joseph Smith, see chapter 4, A Different Jesus, in Carol Hansen's book Reorganized Latter Day Saint Church: Is it Christian?.
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Un-Christian Priesthood System. The RLDS church is governed from top to bottom by a complex hierarchy of priesthood offices designed by Joseph Smith. While the name of each of these offices can be found in the Bible, not all of them have a rightful place in the Christian church. Rather, we discover that the entire system is actually blasphemous when we examine it closely. (See the next question, below. For an in-depth discussion of the entire RLDS priesthood system, also see chapters 11-16 in Part Way to Utah: the Forgotten Mormons.)
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Claim of Exclusivity. While some RLDS would back away from this belief, Joseph Smith taught that his was "the only true and living church upon the face of the whole earth" (RLDS Doctrine & Covenants 1:5,e). He denounced all other churches as an abomination to God, and insisted that his priesthood alone has authority to represent God on earth. Rejection of others and claims to exclusivity are common earmarks of cults.
Paul Trask