I mean BYU might arguably be the helmetiest team in the new Big 12, and they are an elevated mid major. Yet their competition for the honor is OSU2 and West Virginia.
If they joined the Pac 12, they'd be behind Oregon, UDub and Stanford in prestige at the very least, and there are probably a few others that are ahead of (or at least equal to) OSU2 and WV's prowess. Plus they'd have four other teams in their own time zone, instead of being an extreme geographic outlier in a conference that contains the prestigious brands of UCF and Cincinnati.
I mean, come on. The choice is obvious.