I just got around to watching it, and I thought it was terrific. Why did it fail? Because people did not understand what it was about, and the advertising didn't help. People wanted another action movie, and not something with a message, and a Christian one at that. I'd like to see it shown for high school and adult church groups, but beyond that, it isa movie that should be seen by a lot of different people, because it's message is relevant to all of us.
If we all liked the same movie, there'd only be one movie!

because it sucked?

Old news say: "Finally, Machine Gun Preacher is currently slated for a nationwide expansion on Sept. 30, though that's probably contingent on its reception during its limited opening a week earlier."
So I guess the answer is: It didn't do well in the limited opening. They probably realized, that they have a movie with no real target audience in their hands. It's too shallow and cheesy to be an intellectual/political thing, too brutal and disillusioning as a family flick, on the other hand for an "African Rambo" the action is too scarce and slow-paced, and as a personal drama it's too uneven and all-over-the-place to really work. Bottom line: You don't have to be an expert to know it's a difficult movie to promote. It may seem weird to kill it completely because of that, but that's the game nowadays: Bad predictions, no release. They just go for the DVD and TV market to reach break-even. Which is with 30m budget not that unreasonable. In the end, it's not that much money anymore for a movie..