FF know that a new rulebook, for their largest 40K RPG would sell, and would sell well, they don't need to wait for sales of the core rulebook to drop to literally 0.
Disclaimer: I have absolutely zero inside knowledge in this regard. I have never to date worked on any Dark Heresy product beyond my early playtesting work for Black Industries, and I am not privy to any information about Dark Heresy in its current form that is not available to the general public..
Is it the largest, though? Sure, it's been out the longest and it was released first, but beyond those facts, is there any reason to assume that it's more popular than Rogue Trader or Deathwatch or Black Crusade? Especially given that games with extensive arrays of supplements tend to dissuade new players because of the sheer weight of available material that may or may not be necessary.
Beyond that, if it is selling well, and is consistently popular… what is there to gain by risking the alienation of that existing customer base to bring out a second edition that would be less compatible with the piles of material they already own (because no matter how much you argue, "updated and improved rules" does not sit well alongside "perfectly and flawlessly compatible with everything that came before").
A new edition is a big undertaking with many factors and variables to consider. It's a question with no right answer, but rather one determined by a comparison of risk versus reward.
Writing Credits for Fantasy Flight Games: Into the Storm, Edge of the Abyss, Battlefleet Koronus, Hostile Acquisitions, Black Crusade Core Rulebook, First Founding, The Jericho Reach, The Soul Reaver, Only War, The Navis Primer and Ark of Lost Souls