Final answer:
Brigham Young was given the nickname 'the great colonizer' because of his role in leading the Mormons to establish settlements and communities in the western United States.
Step-by-step explanation:
Brigham Young was given the nickname 'the great colonizer' because of his role in leading the Mormons to establish settlements and communities in the western United States, particularly in Utah. After the death of Joseph Smith, Young became the leader of the Mormon Church and guided the Mormons on their journey along the Mormon Trail to the 'promised land' near the Great Salt Lake. His efforts in organizing and leading the migration of thousands of Mormons earned him the nickname 'the great colonizer'.