"why not just send commentators to reddit, HN or another social site?"
Because I want full control of the comments, don't want my users to go to another site (because I don't control those sites, nor the comments the user leaves there, and don't want my users or their interest in my site or whatever I post to be tracked by those sites either).
Because I want full control of the comments, don't want my users to go to another site (because I don't control those sites, nor the comments the user leaves there, and don't want my users or their interest in my site or whatever I post to be tracked by those sites either).