in fact Yewtah has drew higher ratings, but a lot of that had to do with the opponnent and circumstances Yewtha was playing. For example, byu was playing OSU one year and Utah Cal, who has a larger following. BYU has also been consistently playing in one of he early bowls of the season and Yewtah a bit later.
I think there is no question that Yewtah fan base is growing, but byu's is still bigger, especially outside of the state.