since I have a lot of family from Nebraska I've heard quite a bit about this.
the main thing apparently was Tom Osborns retirement, the program never recovered after that. None of the head coaches hired since have put together a successful football program and none have lasted. Whether they had long enough to build their own program or not is up for debate, but NE fans expect a degree of success, so if the coach can't deliver they get the boot.
One of Osborns former players who became a coach in his own right Scott Frost has returned in the last couple years as head coach and started to rebuild a successful program. Maybe you will get a good fight again soon