The related articles and comments are not rendered until after the page is readable. If they had to be rendered on the server, it would cause the article to take longer to be readable.
There's a lot of different caching and persisting strategies you could put in place to load comments or related articles quickly that don't involve entire HTTP round trips. There's a definite trade-off there (and a not insignificant one, especially on mobile)
Not to mention, HTTP2 mitigates a lot of this, without the downside of huge upfront downloads.