BYU has great home attendance...so what? Home attendance doesn't necessitate a national audience or potential national revenue. How does that home attendance translate to revenue for the conference? If it won't appreciably increase conference revenue, then it is an irrelevant point in the conference expansion discussion.
If the addition of BYU will increase the revenue of the other ten Big 12 teams, they will be considered for expansion. Otherwise, it makes no sense to add them. There is no magic stability brought to a conference by having 12 or more teams. These days, stability is brought by money. If the Big 12 can negotiate a lucrative TV deal with only ten teams, then the conference will stay at ten teams. If it is determined that 12 or more teams are needed, then the conference will expand.