How to find out who were the reviewers of an article?
Although @xLeitix answer is spot-on, I think you are missing the main point. Anonymity is directly linked to any democratic process. Likewise, your election vote is anonymous. In this sense, a review is just a vote of confidence for the reviewed article and therefore it has to be and remain anonymous for a more objective opinion. 

Another point is that anonymity in reviews not only protects the reviewers against repercussions but also protects against nepotism and mutual exchange of favors. Moreover, anonymity also ensures that all reviews are (almost) treated equal. So, a favorable, short review from a professor (who just said ACCEPT because he might personally know the authors) might count less than an informed, in-depth review even if that review comes from a PHD student. So, although the editor knows the reviewers, one reviewer cannot argue / discard with the other reviewer based on their individual status. In this sense, anonymity also protects the reviewed, since if the paper is actually good, it is more probable to be reviewed based on its merits then the authors' public relations.