User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-5556688-20150519134530/@comment-126761-20150519135809

A major reason is that it's not good to have articles that completely change layout on different devices - it makes it very hard to keep them looking good across all devices. We also have to consider a much wider range of screen types than existed in the past. Added to that, there is a reasonable limit to readable line length - once you go beyond a certain point it becomes harder to read an article.