I wonder if the film might not have done better if they had made a more faithful adaptation of the graphic novel, which is a metaphor for the conquest of the Native Americans by the white man. With the cowboys of the title standing in for the Native Americans and the aliens in the title standing in for the white man. One never knows.
That theme is in the movie. I don't know if it's as blatant as it is in the graphic novel (which I haven't read), but everyone in my viewing group saw the metaphor.
Hard to say, as I haven't seen the film. Only the trailers for it, which did nothing to make me want to see the film, and the metaphor certainly wasn't in the trailers. But, what did do they say about trailers: "TAL! Or Trailers Always Lie!"