Why is a Mormon settlement called a "colony" while other settlements are not?

Solution 1:

I believe it's because these communities were considered offshoots of the primary Mormon settlement in Utah (after the Mormon's migrated from Illinois). Wikipedia describes this process:

The cooperative ethic that Mormons had developed over the last decade and a half became important as settlers branched out and colonized a large desert region now known as the Mormon Corridor. Colonizing efforts were seen as religious duties, and the new villages were governed by the Mormon bishops (local lay religious leaders).

This is similar, but on a smaller scale, to the way Europeans colonized America and other parts of the world. While the Mormon colonies had local bishops, they presumably were under the control of the leadership in Utah.

There's similar hierarchical organization in other churches (e.g. Catholic diocese, with Cardinals and Bishops), but they weren't formed deliberately by communities moving into these regions. Instead, they formed after the fact as members of the church who happened to live in the same vicinity organized.