User blog comment:Lancer1289/More information about the new skin/@comment-1691295-20101001072343/@comment-1691295-20101001091636

1024x768 is the lowest common resolution. Take away 300 for the sidebar (necessitated by having ads that don't interfere with the content), and you're left with 724. Take into account some padding and the fact that you don't want to have the content jamming right up against the browser window and you're left with 660 for content.

Blame low-res screen users. It's not like we shouldn't be making layouts that work at that resolution already anyway.