People don't like flex in the stems and bars, I think carbon quills gonna feel like riding a spring.
Also, production cost (plus development of manufacturing processes) could be huge ... for demand fairly low. I mean it's gonna be uber expensive and most people would opt for age-proven aluminum (or CrMo) stems for reliability (at the cost of minute weight saving). This leaves very little incentive for companies and factories. ... You ain't polish my shoes for free, right? Nothing is free.