Ziggy wasn't drafted because of his on field production. He was certainly good in games, but was never the best player on the defense. He was drafted because he is an athletic freak. BYU generally doesn't get these types of athletic "freaks" when they recruit.
Why before and not now? Missions hurt (we have 3 times more RMs today than we did in the 80's), the honor code hurts (if you believe McMahon would have lasted at BYU today you are fooling yourself big time) and the lack of an innovative offense (our offense helped in revolutionizing football as we know it today).