Here's a video about someone going over theories and "information" they have on why Inhumans might've been moved. Speculating there's a chance Marvel might be in talks of acquiring the license usage (sharing like Spiderman) for FF4 properties (primarily villains) and to a lesser extent possibly X-Men.
Granted none of it's confirmed, but apparently something big is going on in Marvel for them to have moved the Inhumans date and no one knows what, but this guy seems to have
signs pointing towards that (again don't hold your breath, but if it happens could be nice).
Despite the first part of the title do notice it's a theory and speculation video.