Anything more is speculation.
And the fact that millions and millions have said this is powerful evidence as to the effectiveness of AA for many (but not necessarily ALL) people.
Anything more is speculation.
Of course, this is true. But even at that, it's pretty powerful. Decades of research doesn't seem to have created any alternative that has supplanted AA as the dominant resource for alcoholics. Maybe some day there will be one. It doesn't really matter.
I believe it's worth noting that AA addresses much more than the physical addiction. I'd say that I regained much of my moral character after working the steps, except I didn't have any to start with. So, I developed my personal morality through the process of working the steps.
Also, another factor that is not measurable is the depth and value of the fellowship. There are certainly all kinds of people in recovery, but it's startling that I have an immediate trust and affinity for just about everyone I've met at a meeting.
I wouldn't be so bold to say that it is the best way, or the only way. But, for me and many others, it works, it really does.
And that's all that matters.