You'll notice newspapers use columns and do not extend the text all the way left to right either. It's a typographical consideration, for both function and style.
From a functional standpoint: Having to scan your eyes left to right a far distance to read makes it more uncomfortable. Of course, you could debate this and I'm sure there are user preferences, but this is the idea behind limiting the content width.
From a stylistic standpoint: It just looks “bad” if text goes all the way from the left to right because the paragraph looks "too thin" like "not enough volume" and "too much whitespace." It’s about achieving a ratio of background to text that’s visually pleasurable. With really wide widths, paragraphs can end really early on the left, leaving their last lines really “naked” where you see all this whitespace inconsistently following some paragraphs. I can't really explain why this looks bad any further though. It’s kind of like picking colors combinations, the deciding factor isn't any rule: it's just "does it look pretty?"
In the case of the site in question, the content width is really small. However, if you notice, each paragraph has very few words so it may have been tightened up for style reasons. I would have made the same choice.
That said, if you have to zoom in 170% to read the content and everything else is not also tiny on your screen, it may be bad CSS.