Well, projectors use xenon arc lamps, and their bulbs are ridiculously expensive. Like $200 just for the bulb. That alone would be prohibitive, but their spectrum is also not ideal for plant growth. They release a lot of light at UV, near-infrared, and green frequencies, which are essentially wasted by plants. Plus, the power regulation for a xenon bulb is pretty expensive and elaborate and will cost a lot more than a HPS/MH ballast. Xenon bulbs do have low-ish power consumption, but it's no better than other HID lighting, like HPS or MH.