I think this is because coaches overcompensate for what they don't know about one side of the ball by overdrafting on that side of the ball? I'm not entirely sure, just a guess.
Unless you get a bonafide Guru and in my lifetime only two come to mind, Bill Walsh and Hank Stram, great HC's like great CEO's usually have huge ego's and dominating personality's that permeates the entire organization to the point of leaking out of the walls, toilets and water fountains. These guys literally shape the direction of the team and make them great through the force of their indomitable will. Guys like these have a philosophy of football that they absolutely believe in and it's unshakable and they attract great coaches with the same philosophy to work for them and teach it every day.
Why? The guys were hired to improve the other side of the ball (their specialty) but when they get there, and 75% of their cap is spent on the side of the ball that is better, they can't do too much.
