Granted, it all depends on how you read it. Even Cain can be taken very seriously, with only an acknowledgement that it's humorous. In its own way it even tones down the most over-the-top elements of 40k. You could take all the writing about everyone working until they die on horrible factory as a more tongue-in-cheek thing - Fifteen Hours takes itself rather seriously, but who couldn't see a tired worker mistyping a phrase leading to huge consequences as a commentary?
The problem is less with the writers than with the fans, I guess I'm saying. The writing may be far more serious now, but it's shifted with what the fans wanted. It's not as if one day they decided to stop writing satire, it was a shift over time - we can see how in WFB the difference between 5th and 6th edition isn't as much humor as heroic or unheroic tone. Trying to gauge the mood? Trying to fit the expectations of a changing culture, or shift as their big sellers shifted? I don't know.