The problem with the valleys in Utah is they are comprised of mountain sediment of sand, loose gravel, and rock fragments. This leads to an unstable surface in the event of earth quakes.
The only way to build a heavy weight structure is to stabilize the soil with stone columns and pile drive steel beams deep into the bedrock. The cost would be tremendous and you would be better off building a new stadium in another location (like where Provo High sits) than trying to do it in the existing location.