Every group wants to feel superior to other like groups. There are other sects that consider themselves separate from Protestants.

I think it has something to do with their belief that there are gods and gods and GOD. If you're a very faithful Mormon and you have multiple wives, wear the funny undies, etc....you can become a god.

Really.

It depends on one's definition of Christianity.

Technically, to orthodox, mainstream Christians, a Christian is one who subscribes to the historic creeds of the Christian church that, with minor variations, most Christians would consider failure to subscribe to, to be heretical. These include things like the divinity of Christ, the virgin birth, the Trinity, and something called Sola Scriptura, the notion that 100% of divine revelation is in the Christian Bible.

Mormonism fails the Sola Scriptura test, as they elevate The Book of Mormon, Doctrines and Covenants, and a couple of other texts to have equal authority to the Bible. And so some Christians consider them heretical. In fact it's fair to say that most of official Christendom must, definitionally, consider them heretical. How most individual Christians feel is more questionable. Most of them don't know or care what Mormonism teaches, and since Mormons believe Jesus was god, are willing to allow them to at least be the crazy uncle in the basement, so to speak, and overlook their pecadilloes.

For their part, Mormons consider the historic creeds to themselves be heretical codifications of longstanding heresies and misbeliefs, and see their church as restoring to the world a true form of Christianity that has been lost since almost the days of the original apostles. Practically speaking, they want to be seen as Christian because it helps overcome the relative youth and minority status of their faith, conferring more perceived credibility upon them. Also, they feel they love the same God and revere Jesus as god as much or more than other Christians, so ... they want to claim the label.

It's similar for other minority groups. Take the Jehovah's Witnesses for example (please). JWs are annihilationsists (you don't go to hell if you don't believe, you just don't go to heaven) and they are unitarian (do not believe in the Trinity; Jesus is literally Jehovah's son). For these reasons, per the Creeds, they are heretical. But they arguably have as much right as anyone else to self-label themselves as Christians. There are tens of millions of essentially non-practicing cultural Christians all over the world, who never go to church or who only show up at Christmas, Easter, christenings, weddings and funerals (name 'em, marry 'em, bury 'em). JWs are generally more devout than that, so why can't THEY be Christians if they want, also?

Well technically because they don't profess the set of beliefs that have come to define Christianity. But increasingly, people don't let that get in the way of letting people identify as they wish to.

There's another reason a subset of Christians -- mostly fundamentalists -- regard groups like Mormons and JWs as heretical, and that is their failure to adhere to a lot of more specific teachings that flow FROM the Creeds. Once you reject Sola Scriptura, you can claim extra-Biblical authority for notions about the afterlife, magic underwear, and whether dead relatives can be posthumously "saved" by the intervention of living survivors, etc. A fundamentalist Christian, especially, would reject these because they aren't taught (or at least readily inferable from) the Bible. A fundamentalist regards that as the ultimate blasphemy.