Re: Why are there athletic players who can't play D, but unathletic players who can?
Playing defense is doing your homework on your defensive covers. If you know a player's tendencies and preferences, it's much easier to stop him.
Also, athleticism could also very well be a curse on the defensive end if you tend to overreact on fakes and subtle movements. Sometimes, less athletic players don't react as much, making it harder for the offensive player to juke him.