In 1847 why did mormons emigrate to the area between the rocky mountains and the great salt lake?

In a country that has one of it's rights as 'Freedom of Religion'. In 1847 the Mormons were not provided that right. The federal government under President Polk refused to protect the Mormons and Mobs & terrorist's burned their homes and forced them to flee for their lives from the state of Illinois. Just because the Mormons had some different religious beliefs than their Christian neighbors.