inactive or ex-communicated remain a fan of BYU because that was the way they were brought up. Females who distance themselves from the church become Ute fans in protest.
That is not in every situation but in 99% of the situations I came across when I lived in Utah.
As for me, being a non-Mormon and brought up a Gamecock, I still remain loyal to the Mighty Cocks but having my degree from BYU, if push came to shove I would want a BYU victory.