Because Lavell was stubborn the last 10 years or so with his defensive coordinators/coaches and BYU had some truly terrible defenses and mediocre years.
I'd also take Lavell overall over Bronco but it's an apples to oranges comparison. In-game I'd take Lavell as an overall decision maker but the overall program has a lot more challenges now than Lavell ever had with honor code enforcement, social media, an in-state team in the PAC and Bronco's schedules have been tougher.