I was thinking about that as well... but I imagine starting at 6 would cause a lot of problems with guests exiting the regular, day park at 6. With such mass traffic, as well as the fact that half of the crowd present aren't HHN guests, you can't really do a Scaremony then, either.
As much of a fan as I am of the idea of a Scaremony, I have to be honest that I haven't seen it in person for the last 5 years because I'm too busy doing all the early entry mazes. And I'll admit I was one of the people silently irritated I was being held back from getting a maze coming up from the backlot to the upper lot, but unable to progress further because we all had to wait for the Scaremony near Special Effects Stage. That was even worse because you have to wait, but don't even get to see anything. Mind you, I didn't state my irritation to guest relations or anything, and I understood, but I was irritated nevertheless. But try explaining to masses of impatient people why they're being help up for no visible reason. And especially when hours are short and mazes are plentiful, delays can literally make a difference between seeing all the mazes or not (without express, anyway).
All that said, I am still a bit bummed because it was fun to watch all those Scaremony vids, and it was something unique to Hollywood.