There are the subtle issues that are mostly unconfirmed rumors like the land the stadium is on being unsuitable for the addition of significant weight to the structure or the massive overhaul that would then be required to get the structure up to modern building codes.

But you also have the obvious issues like the endzones and sidelines having completely different slope angles and 4 big ramps and upper concourse bridges that occupy the space where structural support would need to go.

BYU needs to do a TCU or Oregon State style renovation if they are going to go big. Just blow up one stand at a time and rebuild it, move on to the next.