In blogs people can come along anytime and use comments to add additional information/context/perspectives, point out misunderstandings or outdated information, share updates, pose questions and start interesting conversations that do not have an expiration date on them.
The discussion for the article can be found on the same webpage by readers, they don't have to go looking on external sites, most of which have terrible searchability and now require logins just to view content and can delete threads and valuable discussions arbitrarily.
I just realised while writing this comment how much I miss web comment culture from the 00s.
That said, I run old fashioned forums and some older threads get revived there from time to time with new insights. Others get flagged up by copyright holders under DMCA takedown threats or bumped by spambots though.