[hayleox]

The problem with unbounded page width is that the content often didn't work well that wide. I use a 1440p monitor and on the old theme, paragraphs were often a bit too wide to read comfortably. As a Wikipedia editor, it was also very often a challenge when writing articles – it's difficult to make your placement of images and tables and so forth look decent on viewports anywhere from ~400px to >2000px wide.

All that said, the new theme does allow you to return to wide mode -- on any page on the site, there's an Appearance options panel in the rightmost column where you can switch from Standard to Wide.