X-Men: Apcolaypse made $544 million, which was a shadow of X-Men: Days of Future Past's near $750 million.
I think it was all downhill for X-Men: Apocalypse from the moment they released the first look at Apocalypse who resembled the villain from Power Rangers. Honestly, how could Singer or anybody attached to the movie not know how bad it looked? Especially seeing how a makeup artist on the film DID come up with a version straight out of the comics (see below), which for whatever reason they didn't go with. It's mindblowing.
Then there is the storyline, which failed as well with Apocalypse's limited powers, Phoenix at the end, etc.