The reason for the emphasis on the other titles is because Marvel / Disney control the ancillary film rights whereas all the film rights for the FF- the Fantastic Four — and the X-Men are controlled by Fox who has no interest in the comic books.
I guarantee you that if 10 years ago, when Marvel was approached by Disney, if the X-Men film rights were owned by Marvel Studios and not Fox the X-Men would probably still be the paramount book in the canon.
That has nothing to do with comic sales, that has everything to do with the fact that the film rights are controlled by a rival corporation.
I could wish for something else but it ain’t my 5 billion dollars.