maybe the audience over there is seriously wrong. With Trumpism on the rise, maybe the X-Men franchise does not fit the zeitgeist anymore. Maybe Superman, Batman, Captain America (!), Deadpool and Iron Man are the hyper-masculine superheroes the mainstream United States want and deserve?!
But the movie was also not able to attract the othe side of the political spectrum. Instead of highlighting the central socio-political aspect of the X-Men franchise, "X:A" was also a total letdown to articulate any political metaphorical dimension.The killing of Magneto's family could have been easily presented as a hate crime but instead it came across as an accident and barely showed the bigotory of the local police men (why not relate this scene to police killings of innocent black people in the US?). I felt like the political subtext was consciously diminished to produce a better mainstream blockbuster experience (but still failed to attract any larger audience)...maybe FOX tried to pander to white mainstream America and made the X-Men franchise more easily consumable...
I think this is a serious reach, but I think you're touching on something interesting here.
It seems the cultural zeitgeist in the US is nostalgia and 'fun'. I've seen so many reviews of the CBMs this year, and the 'fun' word keeps cropping up. CBMs are held by the yardstick of being fun, and if they are not colourful or quippy, their esteem go down slightly in people's eyes. We all know how much people are in love with the quippyness of Iron Man, Spider Man, the Guardians of the Galaxy crew, Deadpool especially, etc. People in America it seems, want comic book movies to not be ashamed that they are comic book movies.
I personally find it very strange. In past years being 'fun' was not such a big deal as it is now. I think what's happening is that the cultural zeitgeist now is all about the geek culture, and all the childhood loves, which means people now want things that resemble the cartoons they watched from childhood and the sheer joy of being a superhero that kids had when they watched those cartoons or read the comics. X-Men never had that sheer joy. The closest was First Class, and see how some people claim it to be the only good X-Men film?
As someone who did not grow up in the U.S., I didn't grow up with superheroes and all that and so never had the expectation that superheroes should be 'fun'. And speaking from personal experience, all my friends who similarly did not grow up in the U.S. had no problems with the dourness of movies like BvS. They want to be entertained, regardless whether it's fun or not, with some even saying the comedy in the MCU movies is a little too much sometimes.
So it's interesting. Part of the reason why I think X-Men is declining in the U.S., but still performing solidly in other regions is because of the cultural zeitgeist of 'fun'. The X-Men movies so far have not established themselves to be fun movies, although Quicksilver is bringing a little of that to the franchise. It's telling that he is considered the best part of Apocalypse. Of course this is not the only reason, and may only be a small contributing factor, but I think it's interesting to think about.
Also, the lack of political themes in Apocalypse is down to writing more than anything. Kinberg and Singer have very interesting things to say, but none of that translate onscreen.