I'm guessing it's due to them just recently released a 30 y. anniversary TPB, will release a Black Label TPB of it in June and then after that a new HC "DC modern classics" version of it.
I'm guessing they'll wait for a bit releasing it. Perhaps a bit closer to the upcoming Batman which is in the making.
But yeah, I was really looking forward to it. I'm hoping now they'll include Last Crusade when it's -eventually- out, as well as releasing Master Race as an absolute as well. Making up for the delay