Welcome to SR, pjm!
From a logistical/grammatical perspective, it's because the Steps are written from the perspective of the men and woman who had gotten sober "in AA" up to that point -- which, of course, would be the first person plural perspective.
On a deeper level, it is widely believed that the choice to use that particular perspective is important because it emphasizes the fact that AA is a "we" program, i.e. that it is a program that is based on the idea of alcoholics overcoming their isolation and working with/helping each other to solve their common problem.
In addition, given the "content" of the first Step -- admitting the unmanageability of one's life and one's total powerlessness over alcohol, it's kinda important/comforting to know that one is not alone and that there are others who have been there before one and found a way out.
freya