LED's and metal halide (HID) are very similar in efficacy (lumens/watt), but LED's do have the edge. The major issue with LED's is that most of the light bar makers aren't using the newest and most efficient LED's, and they aren't using the best optics for distance. There is a lot of information out there on the difficulty in getting an LED to "throw", due to the fact that an LED emits light from a relatively large surface, whereas an HID or other filament bulb produces the light from more or less a "point source". Search around for LED and aspheric optic and you'll see what kind of distance is possible.
Aspheric optics take up space though, and people want a light bar with 8 gazillion lumens and don't really care if it's all foreground light or not. A good light bar design would probably include several aspheric lenses for distance as well as more traditional optics/reflectors for fill. It would take a lot of R & D to get a really good beam pattern as you have to design multiple beam shapes to interact with each other and spread the light out. It's a lot more profitable to just put a bunch of LED's in a bar and call it a day.