Having served as an elder, I will share my experience. For some reason, among all the potential crimes, heavy alcohol use is something fairly well tolerated in the organization. Even though the bible lists "drunkards" right along with "fornicators," you can bet your sweet bippy the latter is given enormously greater scrutiny than the former.
I think the issues are these...
-The lines between drinker, heavy drinker and drunkard are not always obvious. Elders, who usually drink a bit themselves, are often hesitant to make such a call.
-Has a person's drinking become a substantive issue in the congregation? If the local friends seem relatively unconcerned about it, generally the body won't do anything.
-Is there an unusually Pharisaical elder involved? If so, all bets are off, and anything can happen.
I think much of the toleration comes--not only from elders but congregation members--who themselves enjoy drinking. It's not necessarily verbalized, but I think people feel, "It's not easy being one of Jehovah's Witnesses. We give up so much and are under so much pressure. At least we get to drink!" Any publisher in the position to turn in someone for drunkenness usually does not, thinking that at one time or another they themselves have been just as (ahem) schwilly. And elders? Look, many of them drink quite a lot. And while I'm not suggesting that they are predominately alcoholics, I do think many of them are--and many more are not far from it. Still others are surrounded by these ones--their good friends. Because you yourself are party to a fair amount of boozing, you tend to overlook "heavy drinking" a bit--in most situations.
Yes, there are exceptions, and, yes, officially "drunkenness" is grounds for disfellowshipping--just as fornication is. But a person involved with fornication will be given extraordinarily greater scrutiny than one who got drunk at a Witness gathering. To be found "repentant" of fornication is not easy. To be found repentant of occasions of drunkenness is really easy--unless there are accompanying circumstances.