If a book is primarily fluff, story, and plothooks... you don't need a "5E" version of it because the only stuff that would be edition specific would be monsters. And we've already gotten 3 books worth of monsters so its not like they have skimped out on us there. But a "focused release" book or product like Chronos96 was suggesting they wanted (a la Frostfell, Sandstorm, or Stormwrack) is rather unnecessary if everything in those l is edition-neutral and applicable to any kind of adventure or campaign prep.
I think that's the point of WotC's decision-making process on how they are putting their current books together. They are releasing books that have stuff for players & DMs that aren't just carbon copies of past books they've already made because they want to make these books applicable to a larger amount of people, including those who already own all those older books. I mean, do we really need a new "Manual of the Planes" that just describes and details the same exact planes that they already described to us in 4E, 3E, 2E and so forth? Personally, I don't think so (although I'm sure a few people will say "Yes, of course they do!".
But when we have entire books dedicated to the Feywild and the Shadowfell that they wrote for 4E, what would we gain by them just virtually reprinting them and slapping "5E" on the cover? A bunch of extra monsters in 5E styled statblocks, sure. But we already got a bunch of monster statblocks just that in Mordenkainen's Tome, the only difference being they chose a new part of the story and fluff to talk about (the Blood War), rather than the same "Here's four paragraphs about Limbo" that we've received many times over. If they are going to release books mainly filled with story, fluff, and hooks meant for only one specific type of player (DM or otherwise)... they aren't going to just re-do ones they've already done and which all of us could purchase and use right now.