Why didn't somebody do something? The other way is not wrong. Why did somebody not do something? Putting the not outside of the normal order causes more emphasis to be placed upon it, a rhetorical figure known as anastrophe. From NOAD: anastrophe |əˈnastrəfē| noun Rhetoric the inversion of the usual order of words or clauses. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.