I actually quite like it though. Takes less eye movement to traverse along each sentences.

I think it's a mistake, it's due to:

    .skinny {
        width: 30em;
    }

on `body`. To me at least `.skinny` suggests there's a non-skinny style it should be picking up on non-mobile/small screens, but isn't.

edit: Ah, yes, there is also a `.wider` (not being used) at `45em`.

I like it. I can Zoom in 300% and easily read the text with my outdated prescription glasses.

To answer your question directly: because it's more legible and follows sane typographical practices.

Also: why in the nine hells are you critiquing the design of the site instead of talking about the content of the article?

About the same as the main page of this site.

mobile first! :D