I think the lacklustre promotion may be a more significant factor than the lack of star power. While there were two trailers and a bunch of bts videos, it all came in a bit of a rush not long before the film was released. That plus for those not closely following the film, it flew pretty far under the radar with none of the kind of viral marketing that can raise awareness of the film. To top it all off, I only saw a few TV spots for the first two or three days after release. Didn't notice any billboards, posters or the like. While I'll be the first to complain about omnipresent advertising, it does make a difference.
I'm assuming the advertising budget just wasn't that substantial, probably because the film wasn't expected to do very well (hence the release date) which of course can often become a self fulfilling prophecy. Add to that the 18 certificate and some possible audience 3D fatigue...the odds were alway stacked against this one.
Personally though, I'd take one *good* film over a whole franchise of creatively compromised ones.