1. The GM has no financial incentive to excel. Mediocrity makes him as much money as excellence.
2. The GM is bad at negotiating contracts with existing players and free agents, leading to salary cap problems, leading to lack of roster depth and severe dropoff in talent past the starters.
3. The head coach lacks game sense -- i.e., the ability to scheme an offense so that the 4th quarter belongs to him and not the opponent.
4. Mediocrity breeds mediocrity. Have a really bad year and you get a high draft picks and the chance to stockpile high-grade talent. Draft in the middle and get leftovers every year.
5. Poor conditioning, leading to an abundance of injuries, including an epidemic of pulled hamstrings.