Well dare I say it but I can think of 2 or 3 things that have been significant, or maybe those two things are somehow the same:
1. The dreaded "style of play" question - did Ulster football become too obsessed with an outdated style/approach/tactics, and hasn't caught up with advances made since the end of the last decade?
2. Where's the brilliant younger coaches coming through? MH still with Tyrone, PMcG was still prominent, Geezer hasn't really progressed, Paul Grimley, one of the best innovators, went south (pun intended), and the others are either tied to weaker counties or haven't emerged.
Or 3. The investment didn't develop either skills or/and physicality enough to produce the top teams, and/or the drain of lads to other sports/other countries has taken away too much talent.
Another possibility could be just a loss to some extent of the hunger and drive for success, which again could be tied into one or two things already mentioned above. You know, that culture of success.