Why has Inhumans been cancelled?
Word behind the scenes is that Kevin Feige never wanted to do an Inhumans movie, that it was more a mandate by Marvel Entertainment CEO Ike Perlmutter, who he reported to at the time, as a stand-in for not having the rights to do X-Men movies.
Not too long ago, there was some internal conflict between Feige and Perlmutter over the direction of the MCU and actor compensation when Captain America: Civil War was in development. At that time Feige almost quit. The result ended up being that Disney now has Feige reporting directly to Disney Studio's Chief Alan Horn, while Perlmutter controls the Marvel TV universe. It is believed that Feige pulled the Inhumans since he no longer has to accommodate Perlmutter's agendas.
There are also rumors that if the collaboration between Sony and Marvel Studios goes well, Fox might be interested in allowing the X-Men to be part of the MCU. So Marvel might be waiting to see what happens in terms of that.