The reason...
...for this is because it would butcher XtremIO sales, which does it's data reduction in-line like most modern AFAs.
The all-flash VNX is intended to provide the same feature set as the disk-based and hybrid VNX systems so existing customers can have an all-flash option and manage with the same toolset/skillset and same features (RecoverPoint, VPLEX, ViPR, etc)
I've seen even smaller VNX systems with MCx code and a bit of flash deliver some big IOPS with consistently low latency considering the requirements of who is buying them (and the latency of the old VNX with large flash pools), but they are intended to be a multi-purpose, multi-protocol Swiss Army knife of storage with disk foundations, not a hyper performance all-flash thoroughbred.