While X-Men: First Class wasn't the most successful film in the franchise, it basically revived interest in the franchise after two straight failures in X-Men: The Last Stand and X-Men Origins: Wolverine. It's also the only film that Kingsman director Matthew Vaughn directed in the series since he left shortly after First Class and now we know why; Fox wanted his third movie before the second one he planned.
Speaking with ComingSoon, Vaughn revealed that he had planned a second movie that set between First Class and Days of Future Past since the latter was supposed to be the third movie in a planned trilogy. However, Fox wanted to do Days of Future Past after reading the outline and Vaughn wanted a second installment beforehand, which eventually led to him leaving the franchise.
Admittedly, Days of Future Past was one of the best films in the franchise and is arguably the best X-Men movie, depending on who you ask. It was funny, had creative action sequences and a surprising amount of stakes, which is why it's so fondly remembered. Still, it's easy to see why he was frustrated since he clearly had a plan and they didn't listen.
X-Men: Days of Future Past is now available on Blu-Ray. X-Men: Dark Phoenix comes out on June 6.
Read:X-Men: First Class Director Wanted Tom Hardy For Young Wolverine