I don't think population is that important. It's other factors which make horror very hard to pull off.
One of the major factors, also, is that in many ways being able to pull off true darkness is playing off of helplessness or of the great unknown. That's very difficult to do when your character has a bunch of powers on a +sheet which are specifically geared either at making them able to help or know things.
This isn't metagaming but actually roleplaying. If my Mage is a triple Arcana Master or an Elder with vast resources at his disposal then getting him cowed is hard - he wouldn't be where he is if he was easily cowed, and now he has answers; maybe they won't fix whatever is eating his soul but being able to throw a fireball at the bogeyman makes it a little easier than say, a Stephen King novel typical ten year old boy who witnesses the supernatural and not only does he have no tools to fight back, no adult will even believe him.
Add to that some practical considerations. For instance the fact STs sometimes don't know a lot about characters in their plots, or the tendency many people have on MU* to let their characters be extensions of their egos and mistrust STs who take liberties with them and it's not exactly hard to see why this isn't pulled off more frequently.