When you ask yourself why, maybe ask yourself why not? He is an active addict. You have only ever known him in active addiction. The "promise" of sorts (I'm guessing here) is that you would stick around while he straightened himself out. That didn't happen, he wants to continue to drug - you are out.
The drugs will always come first and foremost. They are best friend, thing that calms, thing they must have, everything else (yes, including you and his friends and family) are secondary. If you want to see him fly away from his socializing, have his "friends" say, we really like you but want to see you clean now.
You didn't Cause it, can't Control it and can't Cure it (the three c's). He will quit when he wants to and not a moment before.
I'm very kind, lead a clean lifestyle and only try to help him with my encouraging words and support.
I believe the message is he doesn't want a clean lifestyle and support, he wants drugs.