1) College football programs are money generating machines. BYU's facilities are not on par with the good-to-elite teams. While they may have been great in the 80's and 90's due to LaVell's success, they are now outdated and behind the times;

2) Conservative values have shifted and the honor code is a bigger hangup today than it was when LaVell coached. Plus, the enforcement of the honor code today is not on the same level as when LaVell coached. This meant that BYU's ability to recruit talent (both coaching talent and players) dropped considerably. Can you imagine Jim McMahon at BYU today?

3) Academic standard today at BYU are higher than ever--for athletes too. Was this enforced as strictly during LaVell's era?

4) The local universities weren't competitive back then... They've made significant strides over the past few decades, which makes a small recruiting pool even smaller (e.g., come to Utah and get the "LDS" experience without the honor code).

There are probably other differences, but these four stick out at the moment.