When Marvel first announced their plans for the current slate of MCU movies, Inhumans was originally set for 2018, in between the two parts of Avengers: Infinity War.
Back in 2014, Marvel Studios announced their massive plans for Phase Three of their Marvel Cinematic Universe.
Apparently, Marvel decided that Phase Three was now so large that there simply wasn’t any any room in it for the Inhumans movie anymore.
The only situation right now is that [Inhumans] is not gonna be a part of Phase Three because Phase Three increased in a very good way since we initially announced.