A lot can be said about enjoying the fruits of your labors for a while rather than going to another job to rebuild again. BYU is rolling, they have a staff that is working and players that are performing. New recruits are coming in that have been in the pipeline for a while. While it's rewarding to go to a new job and build something new, it also feels good to see your hard work paying off.
As long as BYU can come close to matching other offers, I don't see any coach on the staff leaving for a lateral position elsewhere.