Does he plan for them to appear in his GOTG book? Might explain it. The MK20 book is too early on to figure out if there's a good answer to the question but there are certainly potential answers. Also, another major Marvel Knights book
spoilers:
end of spoilers didn't have any appearances in the first two issues.
Did he give any more details about his Inhumans story than what you posted? I know people aren't going to appreciate what I'm saying but I don't disbelieve what you wrote. I just genuinely think he got a bunch of other books that took up more of his attention so he stopped giving as much attention to Death of the Inhumans and sort of phoned in the ending.