1. It's still very expensive to fabricate.
2. The manufacturing process is not very environmentally friendly.
There is an alternative--composites made by hemp fiber--that is now getting more and more serious attention. It's almost as strong as carbon fiber, but the manufacturing cost is way lower and is not as bad to the environment when the composite made from hemp fiber is manufactured. We could see at least some body panels made from hemp fiber possibly within the next ten years, which will allow substantial weight savings--possibly as much as 400 pounds off a car now weighing around 2,900 pounds when made with high-strength steel.
Why would anyone want a lighter car when semis blow them all over the highway now?
If in a car crash the fire would burn the hemp fiber and the wreckage would smell of marijuana.....