In some respects the fact there appears to be no threads at all on this forum dedicated to Mars Needs Moms probably answers this question to a degree, but I wonder what happened?
Apparently the Robert Zemeckis film, the latest in his motion-capture 3-D CGI series, tanked bigtime over the weekend, and looks like it could be the 3-D equivalent of Heaven's Gate as a mega-money loser ($150M budget, $7M opening weekend is a flop by any measure, even if one assumes it'll eventually make the money back on Blu-ray).
For my part, I never even heard of this film till it opened, unlike Rango which had quite a bit of advance publicity. I've even heard more about that CG film with the Gray that's coming out (I can't remember the title, though) then I did about this one. And my first impression was maybe this was a remake of Mars Needs Women, the old cult film from the 60s.
The reviews were mixed, but the didn't seem to be overly bad, certainly no better, no worse than Battle for LA or Red Riding Hood, which both did better.
I won't call it the "bubble popper" but is it possible audiences are starting to grow tired of these 3-D animated extravaganzas? I see Rango stayed at No. 2 and I believe that's a good old-fashioned 2-D film. Chinks in 3-D's armor? I certainly hope so as one who dislikes that format, but other than pushback against 3-D I can't really see any other reason why a film like Mars Needs Moms would have tanked so spectacularly.