I find it difficult to read articles that are wider. There are a number of readability studies that back this up, it seems that 50-75 characters (including spaces) is a sweet spot for readability, and that things get terrible pretty quickly once you start increasing the line length. This design is 80 characters wide or so, putting it at the upper limit of reasonable width.

I would rather have to scroll down on an article that I can actually read, rather than fit the article on my screen but be unable to read it well.

Wide text only really works for very short text of a few lines. Long text comments on HN tend to suck pretty badly with wide browser windows.