The simple answer, and the one ultimate reason you can point to for this historic run of incompetence, is that the Bills are terrible at player development.
The only reason I haven't gone crazy knowing they could have drafted Aaron Rodgers if they hadn't moved up for JP Losman is believing that he'd have been driven out of the league by now if he started out here.
They may have had the "guy" many times over here, but they just haven't hired the right people to nurture that talent.