One reason why that might be the case is that HN is a community of people that like to find and solve problems.

Reddit on the other hand is passive entertainment, users are more interested in a novel problem that someone else dealt with.

You could argue that HN, too, is passive entertainment. Many posts are exactly “a novel problem someone else dealt with”.

It's something else entirely. Almost all the posts on /r/legaladvice are shouting into a void. Hopefully it helps, maybe there's some interesting side discussion...

But an update? That means there's actual feedback and conclusions! Something that's desperately lacking on that particular board.

It's not about reddit as a whole.