A lot of these things were either explained or implied in the film. For a start I believe it's heavily implied whenever they bring up "the Westchester Incident," that the X-Men were killed accidentally because Charles had a seizure in his advanced age and the psychic blast killed the X-Men. Logan survived because of his healing factor and Charles I think repressed the memory as well as can't remember do his deteriorating mental health.
They also said in the movie that the reason why no more mutants are being born in the world is because the mad doctor character played by Richard E. Grant created a form of super corn syrup which was then implemented as a sweetener in the world's food supply. It was marketed as just a cheaper sugar product but in reality it was a chemical that was specifically designed to eliminate the mutant gene hence why there are no new mutants being born.
They also said that the adamantium in Wolverine's skeleton being inside of him has been very slowly poisoning him over the decades and reducing his healing factor and now it's just finally starting to catch up with him.
I personally loved the hell out of the movie but I did find these elements depressing.
