The sad fact is that the general public is burnt out on X-Men, especially with how poor the previous two films were. It's just a shame that this excellent film has to pay for the sins of its predecessors.
The general public seemed to like X3 and Wolverine.
Didn't they sell well on DVD?
It seems mostly those that read the comics are the ones that didn't like it.
IMO what has done this film in, is that it lacks all the elements from the comic and the films that made the X-Men great. Instead it goes for a gimmick with IMO brought the X-Men comic down, it's more about who the villain is and cool powers than the cause.