The problem is that those stories are just straight up hard to write.

I mean, break it down from an objective perspective. It's hard to write an unfathomable alien intellect driven by either primal instincts or the vast gestalt intelligence of the hive mind. So that's one side of the story that's almost impossible to write.

Then you have your ostensible protagonists who are going to suffer a crushing defeat and all die. So you've got to write roughly 350 pages of character development, plot arcs, and story knowing that everything they do and accomplish ends in with them getting nommed while also not really having an avenue to tell the other side of that story.

I mean take Devestation of Baal for instance, the Lictor segues were nice, but only because they were small. It's almost impossible to utilize that for more than vignettes.