Because you like everything else about the static blog (static files are easily hosted and trivial to move to other hosts or CDNs, many devs like the integration with git, you like the workflow of doing everything locally until the push to publish, good hosting solutions that scale a lot are really cheap).

If something goes wrong with the server you host (suddenly tons of visitors, some failure), only your comments are down, not your entire site, assuming the static hosting is on other infrastructure (and even if not, it's easily moved again).

Maybe you used an external service before and want to get rid of it, like the example in the article, but do not want to completely change how your blog works for this.