My opinion, no inside information: He was not ready for the big time. Being at UCF enabled him to play the underdog, though he did not play a big boy schedule, he could act like it. Recruiting left over Florida talent is better than Nebraska talent in a faster football world, especially since UNL no longer has a access to the Texas recruiting grounds. Being in the B1G has stymied UNL, who should have looked south or west instead of mid-west. The combination of the above made for a perfect storm in which a decent coach at the junior level, not yet ready for the senior level, excelled, but could not really step up at a school, though rich in history, lacked fertile recruiting grounds for speed to play today's CFB game.
You can probably get a better perspective from any of the several coaches on the board than from me.