Right, the dur-x wood (and technora) constructions have super low warranty rates and that is proof of their strength. The Futuras and bigger Kodes are lighter weight constructions already in wood. So on these bigger and lighter constructed boards, there is simply less weight to loose if you go with carbon while the extra cost would be as high or higher (more area to cover). I don't have any direct insights in how decisions were made, but my guess is that in the Futura range, it was just decided that the rather small weight loss would not improve the boards enough for it to be worth it. The decision might also have been influenced by that in real world testing with iSonics in current wood construction and proto carbon construction, the carbon boards were not faster (and this is the reason why there are no carbon iSonics). It seems the wood construction, though not the lighters around, is still very fast. Maybe Futuras of different constructions were also tested, but this I don't know.
There is also the argument that in advanced wave and freestyle sailing, the board is moved around a lot by the rider and this actually could make low weight (at good strength) more important than in straight line sailing.