There's a good article on the Athletic website about this, saying that Riley is basically putting his final stamp on "his" program (getting rid of the last Stoops).
Here's an excerpt from the article....Stoops should have been canned a long time ago:
The once-vaunted Oklahoma defense has been only occasionally good and mostly below-average since Mike Stoops returned to Norman following his eight-year run as Arizona’s coach. There have been utterly disastrous performances, like a 2012 win at West Virginia, where Tavon Austin — playing his first game at running back — rushed for 344 yards and the Mountaineers racked up 778 total yards of offense. There was a 2016 win at Texas Tech in which the Red Raiders racked up a staggering 854 yards of total offense. There was last season’s 54-48 overtime loss in the Rose Bowl to Georgia, when the Bulldogs averaged over nine yards per rush.