I don't agree with you that any of those movies were actually bad to awful. Well, maybe WOLVERINE.
But I would imagine that the reason FIRST CLASS did not do as well as the other X-Men movies was not because it was the sequel to two "bad" movies, which both made plenty of money (X-MEN: THE LAST STAND grossed $459 million worldwide, and WOLVERINE, which grossed $373 million worldwide) but rather because it was in the middle of a summer of big action and superhero movies, and actually had competition, and featured a new take on the material rather than the one audiences had grown accustomed to, and a cast of relative unknowns to general audiences instead of Hugh Jackman, Patrick Stewart, Ian McKellan, Halle Berry, etc. While James McAvoy has a bit of a niche audience, Kevin Bacon and January Jones are hardly superstars. Jennifer Lawrence had some Oscar buzz, but she was still a relative unknown to most people, as were most the rest of the young cast, including Michael Fassbender at the time.