How and who might have something to do with it but the more important reason is the rejection of parts of the Nicene Creed by Mormons. This is the key baptismal creed for most other Christians.
From https://en.wikipedia.org/wiki/Nicene_Creed
"Some sects, such as the Church of the New Jerusalem, The Church of Jesus Christ of Latter-day Saints, and the Jehovah's Witnesses explicitly reject some of the statements in the Creed, either because of a non-Trinitarian theology and/or a rejection of historically orthodox Christology."
RE: I'm a mormon so I'll clear up some common myths about us