One word... Cost
Just like everything else, when technology is new, its expensive. Eventually costs will come down as more cars begin to adopt them. Right now, only a handful of cars are using high beam HIDs.
Also, HID's require a second or two to light up. Therefore, conventional lens assemblies can't be used for flashing hi-beams. The solution many manufacturers take is to make the hi-beams always-on and install louvres to cover and uncover the lights quickly. This further adds to the cost. Headlights with both hi and low-beam HID's are often called bi-xenon.
I think it's a reasonable trade-off to use conventional halogen hi-beams since they're on so little anyways.