I can see where you are coming from. But I think the evolving fluff is a key part of this game. I mean a year ago, nobody had any idea who any of these armies were. Now, for the ones that have appeared in the books (campaign or novels), and the ones that have gotten Battletomes, we have a ton of awesome information. I personally think it's kind of cool to have the realms opened up to us a little at a time. Also, I'm not entirely sure if they care if we buy the ranges that haven't been updated. I'm guessing those lines have made their set up costs back, and AoS is selling better than WHFB, so their good on that count. See link below for more on this... Assuming the source is any good. Also, the last thing they want is more people reading the fluff for units that the crunch isn't settled on. I mean, they really don't need anyone else rage quitting this game. So less fluff - less interest in the armies - less people upset when they find out their army doesn't really work together the way they thought.
-- http://natfka.blogspot.com/2016/08/how-is-age-of-sigmar-doing.html
And of course the simplest, and most likely truest answer IMO, is that they don't actually have enough of the fluff settled for those factions to actually tell you much more about them. Most likely those factions are still pretty fluid at this point. No use telling us things that are probably changing fairly regularly.
That all said, I'm starving for more fluff, especially about the various undead factions, so I don't fault you for being upset.