Beyond my pay grade but I'll take a stab (meaning I'm talking out of my ass).

Some in fact have but the majority? Probably laziness, but laziness is just misaligned incentive-goals.

Communities have very little incentive to de-reddit. It's actually a huge amount of work and they gain almost nothing directly.

Separately, I was thinking you know HNews is pretty immune to this problem because we don't have a central theme or something, right?

But no, that just means I can't see how I'm being monetized is all. Blind leading the blind.