Needs a big manufacturer to take it on for it to become cheaper. Requires economies of scale as Immy says just like LCDs, digi cams, and any other technology for that matter. Sure Nissan will be doing some good stuff in this field in a few years.
I must admit I would miss the sound. I see a time in the future when tracks will be filled with leccy cars, that will be a weird site to see them all zipping around making no noise!