The end of the SWC (in 1995) was a lot of Pro-Style 2 back teams. When the Big 12 was created out of the Big 8 and the SWC, the big boy on the block was Nebraska. They famously ran a merciless option attack that was power and more power.
In order to be competitive, defenses had to deploy massive DTs and huge, hulking LBs. The secondary's emphasis was on making tackles and stopping sweeps.
When Mike Leach arrived to be the Sooners' offensive coordinator in the late 90's, he went completely the other direction. he wanted small, fast receivers - and a million of them! Early returns weren't good. It took too much coordination between passers and catchers to pull it off, and collegiate players only had 4 seasons to get it right. It was just too difficult.
Leach, for all his wackiness, really is a very intelligent guy. He devoted more time and thought to the problem. He put together concepts that pulled apart defenses, showed his QBs where the vulnerabilities were, and had his receivers run to them. Since "dime" coverage at the time was a silly notion only deployed against 4th and 20 situations, no one had enough defenders to even slow that machine down!
Please note that a lot of these concepts rely on the inherent advantages that the rules give to the offense. On a "behind the LOS" screen, the receivers can block. You can't block them, though, unless the pass has been thrown. Motion tips off zone or man. Essentially, it was just a given that these offenses were going to score - a lot!
The kids loved this PlayStation type offense! The high schools and QB camps began teaching these concepts so prospects arrived at college knowing how to throw a 20 yard back shoulder fade. The talent was going to go to schools where they could shine. Either you got with it and recruited them, or you were going to stand by helplessly - on the wrong side of a 52-10 route.
Big 12 schools would love to play defense. It's just easier to adapt a 6 man back end defense to stopping the run than it is to find a 4 man secondary that can shut down 5 WR and a QB trained to drop balls into a 10in window from 30 yards out.