I can understand yield may be an issue with these screens, but the DPI isn't. It's lower than a 1080 5 inch screen. I think one big area holding them back has been chips that can drive that resolution smoothly within the tablet power envelope. We are only just getting to the point where top end mobile chippery can drive a UHD resolution GUI at acceptable framerates. There has been one pseudo sighting of a UHD Tablet running on a snapdragon 810 but we have no idea how goosed the chip was nor the frame rate involved as it was just playing video which ( assuming a hardware supported codec) is usually less demanding than playing a game or even navigating the standard launcher with the transitions Etc. I have noticed that with the same chips even the transition from FHD to QXGA to WQXGA results in a marked drop in performance, the former having half the number of pixels as the latter. Going to UHD doubles the pixels again. It's going to happen, the next tranche of chips can do it, but they are still just starting to arrive at the engineering sample stage.