The Spurs are coming into this season with major expectations. That's not something they're really used to, and it can be detrimental to the psyche of some players/teams if they aren't mentally strong enough. It seems silly when you say it out loud, but some players never reach their peaks because the expectations are simply too overwhelming.
It puts pressure on their shoulders that they're unable to handle, and the result is repeated failure. Some guys overcome it if it happens to them, and some never do. Kevin Martin reportedly would intentionally stop scoring the ball during his career if he was playing too well.