The salary cap is definitely to blame for parity. The problem is that you can only afford a handful of super-star players, so if one of them goes down with an injury you're fucked. You can't afford to keep high quality back-ups around. The season comes down to your big players staying healthy because your team is overly dependant on them.

That being said, good coaching is underrated IMO. Just look at the Cowboys and the turn around they've made.