The same way he got his Affirmative Action Nobel Peace Prize.
I don't have time to look it up but I seem to remember reading that there were 19 candidates and that the vote went well into the night. There was a tie between two other candidates (not Obama) so as a compromise they went with Obama (affirmative action - give away).
The two top candidates got screwed.
Here are two other articles:
Ever Wonder How Obama Became President of the Harvard Law Review?
http://www.freerepublic.com/focus/f-news/2600176/posts
Until the 1970's the editors were picked on the basis of grades, and the president of the Law Review was the student with the highest academic rank. (snip) That system came under attack in the 1970's and was replaced by a program in which about half the editors are chosen for their grades and the other half are chosen by fellow students after a special writing competition . The new system, disputed when it began, was meant to help insure that minority students became editors of The Law Review.
Law Review Debates Affirmative Action Policy
http://www.thecrimson.com/article/2006/6/5/law-review-debates-affirmative-action-policy/
Prior to 1981, law students could join the Review either by being among the top five students in their first-year sections—each class used to be divided into four sections—or through a combination of their grades and their scores on an annual writing competition, a process designed to preserve absolute objectivity.
But the 1981 editors felt it necessary for their admission policy to take into account the underrepresentation of minorities and women.
Under their modified plan, the top four students in each first-year section would still be elected to the Review, but the fifth spot would be reserved for the top-scoring minority student among the top 25, and if no such minority student existed, the fifth spot would go to the woman with the highest grades.
Two days after the adoption of this policy, three editors—including one woman—resigned in protest.
In response, the Review’s leadership convened to reconsider their plan, opting for a non-quota system that would merely take race and gender into consideration. But despite the modification, the Review continued to encounter opposition from students, alumni, and most importantly, from the faculty.
See my post right above where Carol Platt Liebau talks about Obama’s time as President of the Harvard Law Review. In the second link, there is audio of her talking about him as well as an article.