Primarily because, as of this writing, hardware vendors likely don't have a reason to implement such a thing. Mainly because there isn't any benefit to doing so.
The majority of games and 3D software applications are based heavily around the traditional scan-line rasterization model, and would see no benefit (and in fact, likely significant drawbacks) to adopting a voxel-based model.
Further, there are no emerging standards for software-side APIs for voxel rendering that are backed by significant industry powerhouses (like already exist with D3D and OpenGL).
With ATI starting to explore the direction of moving away from industry-standard APIs towards something proprietary (Mantle), and the possibility of VR headset technology actually becoming a realizable commodity technology via Oculus Rift, there is a slightly greater possibility of seeing something like this in the next few decades.
But up until now, and likely for quite some time yet, the cost/benefit ratio for all parties concerned is just not favorable.