While Django/Rails indeed make it simple to implement per-article comment threads, the hard part is not the collection, storing, and display of comments. It's the fact that you're accepting content from people you don't know or trust and publishing it.
- Can people leave comments anonymously? Few blogs think this is a good idea anymore. So now you need a user identity system.
- Does that mean you're going to have your own user management just for leaving blog comments? Probably not. You don't want to have to manage password reset emails, for example. Letting someone else manage that is more sane. Someone like Facebook/Twitter/Google/Yahoo.
- So do you write integration with all of them yourself? Bleh.
- Then someone leaves a comment. Is it spam? You need integration with Akismet or a similarly powerful engine to help stem that tide. Doing it entirely manually is a huge time sink.
- But not all bad comments are spam. Some of them are ascii-art genitals. Sometimes people just use your comment thread as a soapbox for their personal crusades or get into pissing matches with each other. If you have many comments at all, you need a moderation system to prevent them from creating a hostile atmosphere that drives nice people away.
- You have to ensure that people can't use your comment form to inject a XSS attack onto your page, or exploit some security bug in your web framework to take control of your server.
So people save some sanity by just using a 3rd party blogging service entirely (Blogger, Tumblr, Wordpress.com), or embedding Disqus, or putting a link at the bottom of the post saying "Discuss on Hacker News".
Edit: added the "like social networks" example in the second bullet that was in my head but didn't get into the text.