I think they've pushed them back to give them time to rebuild some goodwill after the OGL fiasco.
While there's no way to be sure on the correlation, Keys from the Golden Vault seems to have been negatively affected by the bruhaha (sales wise). I mean, it's doing fine, but its definitely slow for a 5e book. Of course, there's a thing I call "late edition slump" as well - once a new "edition" is announced (OneD&D might not wind up feeling like a new edition, but the market can't be sure of that yet) then the last few books of an old edition usually hit the market like lead. Only later does their individual quality make a difference in how they're regarded. (Sometimes making them rare!)