We had some quality guys but the backups weren't up to par. Injuries happen and you've got problems. So why the lack of depth? It was a combination of things. First this was when BYU was ramping up academic standards for potential athletes which decreased an already small recruiting pool. Second was increased recruiting competition from in-state schools.
Also there was a scheme change right in the middle with the switch from Anae to Doman. Different offenses require different skill sets and sizes.
Losing Jeff Grimes a couple of years earlier was also significant. His loss was felt for several years.