1. The Crowton era and the advent of social media has turned the Honor Code from a quaint, "guideline" for players to draconian shackles. When you are always in the spotlight, you have 30,000 informants watching your every move.
2. Lavell started his career discouraging missions. In the 80's when Spencer W. Kimball said "every male should serve a mission," the paradigm changed. Missions went from the exception to the rule.
3. The NFL is so, so much more sophisticated in finding athletes now than they were in the 80's. BYU players were visible then...when others really were not. Now they find guys in the Polish American Football League and at Newberry.
4. There were 12 rounds in the draft back then.
5. The academics are so, so much more difficult now.

There are a myriad of reasons. What is amazing is that with recruiting classes that tend to the 60's, BYU is averaging in the 30's in the Sagarin rating under Bronco. In other words, BYU is punching, way, way above it's weight with guys that aren't athletic enough to get drafted. Texas got 5 players drafted. We killed them. Sounds like pretty darn good football coaching. Maybe the cone dancing isn't as good at BYU...but BYU punches above it's weight on the field.