Ok, I think there are two reasons people didn't see it, and both are based off of the film(s)length:
1. People, hearing it is a double movie, assume it means the movies combine for a length of two hours, thus one hour each, and figure they don't want to see two shorter films.
2. People, hearing it is a double movie, assume (correctly) that it is over 3 hours in length, and they don't want to spend 3 hours at a movie(s) they knew nothing about before seeing them; look at 3-hour films like LOTR or Titanic: so many people knew the story that they wanted to go see it on the big screen, while Grindhouse, in its own self a pair of fine films, doesn't have that sort of recognizable theme/story that exists in the minds of the viewers.
Now does that mean these people shouldn't have seen it? Hell no! I would recommend anyone/everyone here going as soon as you can.
Far away, across the field, the tolling of the iron bell
Calls the faithful to their knees, to hear the softly spoken magic spells.......