I think part of it was promotion and part of it was the core concept. I saw trailers for the movie, but that's it. No advance interviews, no print ads (until the movie came out, then they were everywhere). Rango on the other hand has been all over the place (TV, trailers, billboards etc.).
Then the concept is well, odd. The title is IMO cringe worthy and the story wasn't explained well in the trailers. Sure, Mars needs mothers...why? Do all their moms suck at their jobs? Did they all leave to start a war elsewhere or something? Without that "hook" there wasn't anything to grab you for the movie.
I'm also not a big fan of the designs in the movie. Nothing "popped" on the technology side and the aliens just didn't seem interesting in any way.
I am sad that Zemeckis didn't produce a winner here. I'd like to see a cool, live action film from him again. It's been too long.