Why some "Christian" groups are not considered Christian by some people...
Many religious groups are considered not as religious as others by people in different sects and denominations. There are many different reasons why this can happen. The main reasons I believe are where the groups get their religious foundation, to show the validity of one's own belief, and because of man's instinct of "I'm right, and you're wrong".
Sometimes there are solid reasons why a group of believers are not considered Christian. In any religion where there are other sources for direction other than the Bible, there will be conflicts. In Christian Science, the founders incorporated metaphysical beliefs and practices into their form of Christianity. The Church of Latter-day Saints has other prophets and figures not in the Bible as part of their core beliefs. To people who believe the Bible is the only source needed to teach the gospel, other types of guides are unacceptable.
Besides the use of other guides as a reason for exclusion, believers would like to have a sense of validity in their beliefs by dismissing those of others. The best way to show that your religion is more founded than another person's is to exclude them from membership into the "Christian" way by poking holes in their beliefs. Many people would start to question their own beliefs if all groups that claimed to be Christian were allowed to do so. The Latter-day Saints for instance, consider Jesus Christ as another prophet. If they are also considered Christian, what does this mean to all the other churches that think of Jesus as Son of God and part of the Divine Trinity?
For as long as history existed one man would try and prove that he was accurate and others were mistaken in their beliefs.