Detmer explained it pretty well in the couple of years he was recently at BYU:

BYU was one of the few teams who were throwing the ball against team defenses that were designed to stop the run. He said that they couldn't use the same plays now as they did then because they were designed to exploit matchups against 240 and 250 pound linebackers that are no longer there.

BYU overachieved in the 80's relative to their talent by having a very innovative scheme that was different from almost everyone else.

It is hard for ANYONE to do that now because video is so ubiquitous and instantaneous. Concepts like RPO proliferate in months rather than years. It is much harder to be unique now than it was in the 80's, because everybody has film within hours.