An old, rich history with their uniforms. Alabama’s design debuted in 1952. USC? 1965. Penn State? 1953.
It’s sad that I need to explain this, but BYU is not Alabama. BYU is not USC. And BYU isn’t even Penn State. Penn State is definitely the outlier in this group, but having a 900 year old coach for forever helps explain why they never changed their uniforms.
Again, the burden is on you. Your only argument is “well Alabama does it, so should BYU,” and that’s a terrible argument.