Carbon fiber is expensive, needs resources (curing oven) beyond what most FRC teams have, and is heavily weakened by drilled holes (oops, we mounted our thingamajig in the wrong spot, time to move the hole). So yes, it's lightweight and strong, but it comes at a steep price.
Besides, this is the year I think teams will want their center of gravity as low as possible. Using a carbon fiber frame only invites them to put more weight in things that raise the center of gravity.
