Well they may release them any time they feel good. I suspect this expansion lasted until the end of this year, and its only may now. I dont think they want to give us something best (there are only 2 items possible to be staggered legendary and mythic, i dont think there will be new arcane, so mythic = best) in may and we keep it until the expansion ends (which is 7 months if it lasted until the end of this year, may be more) because it defeats the purpose of "items staggering"
With the release of the upcoming expansion, we will be staggering new item release over time. There will be less “filler” items. Then, a little further down the road, we will release additional content that hold the potential for different or improved items.

With the staggered release of new gear, you’ll have new things to get excited about more often. Rather than just getting everything there is at the very beginning and a longer wait for something new, we will be offering more frequent content with gear improvements.
I do hope they release something new though, just not the best stuff that we use forever.