Because its just very different (for example mormons don't believe in the trinity which almost every other christan does since 1700 years back).
It's the same reasons as jews don't see christanity as part of judaism: They might have started from the same background etc but since they diverged so radicially from each other they became two different religions and not just different sects within the same one. Since mormonism is very different from other christian sects one might argue that it should be viewed as another religion.
Sure, it's hard to draw the line on when something is just a different part of a religion and when it's another religion. There will always be those who want to see it as similar and those who want to see it as the same.
I would probably say that there are good reasons to argue that mormonism is so different from christianity that they are two different religions sharing common traditions (like judaism and christainity or hinduism and buddhism).
The church used to be even more different from christianity. I read a book a while ago were the professor of religion argued that the early church was more judaism than christianity since it shared a view of god/morality/etc that had more in common with judaism than christainity. This latter changed when the churched moved more to a christian model but still... Btw, this is the book that discusses why the early church was more of a jewish sect than a christian one: http://www.amazon.com/Joseph-Smith-Jesus-Satanic-Opposition/dp/1409406709/ref=sr_1_2?ie=UTF8&qid=1452712719&sr=8-2&keywords=Douglas+J.+Davies+mormon