because it can be humiliating and if not completely dealt with could still cause shame...More than likely though it is a matter of being judged. Once someone finds out a person is an alcoholic or drug addict then they are perceived differently and future behavior can become suspect. but if they don't know then they see you as you are and not as you were.
There is an expression ~ you are only as sick as your secrects ~ meaning when we hide who we are then our behavior is not always rational because we are hiding and trying to protect ourselves. There is true freedom in being honest but there is also a cost sometimes but in my experience the price of hiding is always more expensive than being honest.