The rumour going around, back then, was that Denuvo went under the table and paid crackers who had the ability to break their DRM to like -- Stop doing that. And that this was the reason EMPRESS (actually insane woman) was the only person left who could crack it.

Of course, there is no way to prove that.

I think it is mostly due to it being difficult to hack and those with the skill are well employed and don't want to risk legal action or spend the time to crack games.