Partly that, but I think there are also other explanations.
1: I think Scott Aaronson is a better communicator than Robin Hanson, at least in the posts in question. Having read some of the blog posts that went into the pre-read, and then Scott Aaronson’s summary of it, Aaronson does a better job at summarizing the research, connecting it with our day-to-day lives, and making it more fun to read. It has less detail, but that’s arguably a good thing: I don’t need a huge amount of detail on theoretical models for extraterrestrial civilizations, I don’t have the time or energy for that. It’s interesting, but I don’t want to spend hours today reading multiple different blog posts and an academic paper trying to grok the particulars.
(I personally have a pet peeve in that I feel like the typical “style” of academic papers is very bad for communication in general.)
2: WRT Hacker News and /r/SlateStarCodex, people comment there because they want to discuss it among their “tribe”. Wandering onto a stranger’s blog and putting something in the comments can be an anxiety-inducing affair: you don’t know what the people there are like, you don’t what their rules or (sub-)culture is like. You might not fit in, or you might even piss off the author. Meanwhile, I could probably guess pretty accurately what kind of comments are going to show up in any given popular HN story, and how to discuss things in such a way that it would go over well here.