I'm guessing money played a pig part. I know LSU made Saban the highest-paid college football coach ever at the time. Meyer probably wanted to go somewhere where he wouldn't be left out of the BCS Championship after an undefeated season.
When you look at guys like Saban and Meyer, they always left programs in much better shape then when they started. Riley had a few decent years by Oregon State standards, but ultimately left the Beavers in just as bad of shape as when he started there. Maybe worse. He knew he had a 2-10 level team coming back at Oregon State and may not have survived that.
And maybe it is because coaches know where a program's ceiling is? For all Meyer and Saban's accomplishments, they moved on. They knew they could accomplish more at another program.
History showed Oregon State has a certain level that you can expect to reach and it's hard to go higher. It was a .200 program and so many loved to point out Riley was a .500 coach (.541 I believe). So he had a winning record at a school with a low ceiling. I'd look at Riley a whole hell.of a lot differently had he been a .541 coach at USC vs OSU. But it's easier to ignore the ceiling when you have an agenda to criticize.
It amazes me that a supposed fan base as smart as ours doesn't get the concept of this, when we have one of the few P5 basketball programs with no NCAA basketball tournament wins. John Groce wouldn't even take our call. Why? Because he saw the ceiling, and he didn't think he would or could break through that ceiling.
So when people start talking about accomplishments at places where the ceiling level.is so glarring, like Oregon State football, ask yourself why doesn't Nebraska have a better basketball history?