They didn’t know how to play him. Because of him they figured out how to use Robinson and Clark and perhaps mills.
From what I recall hearing, Casey was too inconsistent in practice to win the starting job. They recognized his skills and wanted to use them, but felt they needed to limit him because he just made too many mistakes in practice. When he got into games, that sometimes was apparent. He was either the most amazing player ever, or a liability. We needed him to be somewhere in between for the majority of plays!
When he was forced into the starting role in the '99 Alamo Bowl and played well, I think that boosted his confidence and the staff's confidence in his abilities and opened the door to whatever consistency he attained. he was fun to watch, no doubt.