I don't think it's the OU connection. For example, if it really had been Bob Stoops being considered for the LB coach, that would be considered a major achievement and he would have been welcomed. Obviously Bob Stoops wouldn't actually consider anything other than a head coaching position, but I'm just pointing out it's not the OU thing.
After reading some about him, by many accounts, including several OU posters on the Longhorn boards-- Mike Stoops is real asshole. And he's also rumored to be a boozer with a drug habit. That's not something the Texas administration would be comfortable with, given Sarkisian's past.