Where is the claim that the Mormons found Salt Lake City first? The Catholics were in Utah before the Mormons, they established a couple of missions here. The trappers were here before the Mormons. And Brigham Young pored over maps after the martyrdom of Joseph Smith to find a place where the members of the Church of Jesus of Latter-Day Saints could emigrate to and not be persecuted and driven out of their homes for their religious beliefs.
Utah conditions were such that development took a higher level of technology and cooperative social organization than could be provided out of New Spain or Mexico.
The LDS were prepared for the experience and followed COTFB into the area.