Struggle to adapt to changing styles of football
Over-reliance on promotion from within
Constant squads full of overpriced individuals and below-par squad players
Perhaps the main reason is our completely catastrophic handling of the Division 1 to Prmiership rebranding. The door was open to show the club to the world-market, using our reputation and recent success to capture the hundreds of millions of potential fans who suddenly had the one of the best leagues in the world being shoved in their faces. Instead we fucked about trying to cling onto our traditions and business models while you played an absolute blinder in marketing, from sponsorship to well-judged pre-season friendlies, even to giving old shirts to charity and exposing your brand to people who may not put any money directly into the club, but certainly did no harm.
Meanwhile we sat on the throne, so confident in our innate ability to stay there we let these new oppurtunities for growth slip past, and only tried rectifying it when it was far too late. So we ended up as a huge club with a huge British fanbase, but an average-sized stadium (for the Premier League) and nowhere near the number of foreign fans you'd expect for a club of our supposed stature.
Doesn't help that the only really, really good transfer window we've had in the last 10 years has been Rafa's first.