User blog comment:BertH/Fluid layout has arrived/@comment-768817-20131205202357/@comment-452-20131206000127

While difficulties in maintaining two separate layouts is often cited, I don't really think it applies in this case, because it is possible to simulate "the fixed width look" using CSS fairly easily.

The reason that's not allowed is probably more to do with the fact that they want all wikis to have the same general layout.