HN users don't avoid humor, but most humor in internet comments is repetitive, obvious, or otherwise a bit lame, so it tends to lower the signal/noise ratio and get downvoted.

This is controversial because people overestimate how funny their jokes are, especially to a large general audience, and prefer to interpret downvotes as humorlessness of others. I think scott_s described this best: https://news.ycombinator.com/item?id=7609289. (Note the phrase, "When you have N people"—it explains many things about HN.)

HN's culture is probably also influenced by Reddit loss aversion syndrome, given that it started as a kind of alt-Reddit (I don't want to say anti-Reddit!) and still has the younger sibling's tendency to compare. As a Canadian I'm tempted to say we're Canada to Reddit's U.S., but I may not be allowed to say things like that any more.