Here are a few reasons why.

1. The way you react to life remains instinctual and involuntary: Despite the work you’ve done with a therapist or a sponsor in rehab, there’s little chance your programmed and instinctual reactions to life have changed much.  The process of re-scripting primary programming takes a long time and a lot of work.
2. The environment that fostered substance abuse in the first place hasn’t changed: So you went away to drug rehab abroad. There, you are taking a serious look at yourself and have begun developing personal awareness around the issues that need addressing.  You may have uncovered several of those issues and begun the healing process.  Unfortunately, aside from taking a break from you, many of the outside factors that triggered using substances remain unchanged and may have intensified in your absence.
3. Home often includes family of origin: If returning home includes proximity to family of origin, you will likely be returning to childhood trauma and other subconscious disturbances that will make the desire to numb out with substances indefensible.
4. You, your identity, and your history: Without question, much of the work you will do in rehab is designed to deconstruct a flawed identity and rebuild a healthier one.  Your previous identity, who you were before you left for rehab, the years of baggage you produced and ran around with are known quantities in the community you come from. You need a new start, a clean slate.  Folks back home are often not inclined to provide that.
5. Time, Practice and Repetition: Time, practice and repetition are the 3 essential qualities to mastery of anything in life.  We need to acquire all three of them to have a chance at successfully executing the radical change our lives undoubtedly require.