Who knows exactly why James. I'm not sure the experts even agree. And I would even question the existence of a "regular drinker."
Sure a certain percentage of people may control their drinking for a period of time, but I suspect they just haven't consumed enough alcohol to have crossed over to the point of no returning to controlled drinking.
But as Scott and others have already mentioned, I no longer dwell on that question. For me, there really is no good answer, and I realize now that the real question I was asking was "how can I return to drinking without suffering any negative consequences."
However, I do understand that some of us are naturally curious and enjoy exploring the reasons why, but I contend, that there is no one satisfactory answer to the question you're answering. And I also suggest that dwelling on that question for too long can impede one's sobriety.