Cause they're fucking nuts. Mormons are so bad ass that regular christianity wasn't crazy enough for them, so they completely re-wrote the story to be completely like the Bible but take place in North America and have anyone non-white as an evil non-believer. And on top of that, they made up this release clause that says anyone can be a prophet so long as you can read, write, and come up with crazy ideas that you can convincingly argue are god's. Which is why we have Polygamy today and mass murders in the 1800s.