I would suspect that the target demographic for sites like this is much more likely to be using ad blocking plugins, adding a further hurdle to monetization.
I block ads, but I have a subscription on LWN. Probably would have done the same on h-online if their comment section would be a bit more integrated. I've been reading this site for various years, the reporting is pretty good.
Why don't we as users make sites like these able to monetize? In a weird way, most sites are the way we don't like them, because in some weird way that's what we "like" and that's what pays off.