In my opinion, that's because Hacker News --- besides self-inflating egos as noted by @petercooper --- has the right tools and already mature community which are more suited to host civilized and fruitful conversations than on a blog.
My aversion for blog comments stems from how little we can trust content there: can I use "pg" as a username on your blog and impersonate Paul Graham? Where do the avatars come from? etc. Lots of little questions of whether or not I can trust what's being written in the comments.
Other than that, there is also the issue of quality: is there a system (e.g. karma, up/downvotes) that prunes the bad comments? the spam?
What about the ever-so-different way the comments are presented on blogs? Are comments threaded or linear?
To me, HN is the perfect framework for good conversations.