One reason is that they need a good amount of power to run them. Turns out Universal Display Corporation has the solution with a display tech called PHOLED [Phosphorescent OLED]. PHOLED is four times more efficient than a regular LED display. Like any new technology, the US military is getting their hands on this first. They already developed eight working devices that were submitted to the Army R&D branch for evaluation and testing.
They were able to build a 4.3" PHOLED onto a thin metal foil, in turn built by LG Display, making a flexible wrist-mounted screen that can display full motion graphics and has reportedly been ruggedized (in an unspecified way); the reduced battery requirements of the technology should lead to truly wearable displays.via core77