User blog comment:Susanolivia/Experience the new Wikia/@comment-93604-20100925120341/@comment-761251-20100925124511

The content width is chosen so that the sidebar ad is visible without scrolling when the page loads for most visitors. If you choose a wider content area (which is easy to do with .css, usually), you'd be obscuring the ad for some visitors, which would be against the Wikia Terms Of Service.