All your coaches may meet your minimum standard for coaching in your regular season league, but that doesn't mean they are all equal.
Many coaches are great at one level of play but have a difficult time transitioning to a higher one.
Another problem is that some regular season coaches of limited ability are blessed with great talent. In my opinion, the policy of letting the regular season champion coach the all-stars is a simple means of avoiding controversy. If the board believes your league champ to be the best, they'll vote him in. A second disadvantage to making your league champ manager the all-star manager is that it doesn't give him much time to plan. Our league has already selected it's all-star manager.