Both HN and XKCD probably have a fairly broad audience with decent overlap. Seems to happen with popular websites, they lack strong specific correlation with anything.
Yeah, seems like a difficult thing to tune with recommendation algorithms.
I think it essentially boils down to the Friendship Paradox. I've mostly ignored the problem as I'm more interested in the fringes of the graph, but I'll note even the likes of Spotify and Youtube struggles with this.
You mentioned rich data earlier, and as an amateur sitting on the sidelines I think that's the key. Once I fed my entire library into LibraryThing, its recommendations were orders of magnitude better than, say, Amazon's, who only knew which books I'd bought through them. In fact they were so good it was mostly recommending books I'd already read, but didn't own.
My toy, which relied on scraping <ul> and <ol> lists of people's favourite bands from personal websites, had almost nothing to go on by comparison. last.fm might have been able to do something good with the data they collected, but they seemed to die on the vine once they were acquired.