Carbon rear ends must be the hardest part of the bike to perfect.All brand seem to do the mainframe first
Definitely, less material to use on the area that under goes the the most side to side and torsional flex. Id also wager its not effective from a price standpoint if they can't drop the weight, Alloy has served us well to counter the stress the rear end needs to deal with, if we don't need carbon for its strength/stiffness purposes in the rear, that only leaves us with weight. If they cant engineer it in a way that retains or improves its stiffness while dropping weight its a waste of resources if alloy does the job and is cheaper.
Equally the front end has more mass to loose weight so its the logical first move, its also high over the COG so dropping weight there is hugely beneficial over the rear end which is low and has less bang for buck on return.
I think we can expect most bikes will shift to carbon though in the rear eventually, but that implies that they have one of the following goals.
1.Drop weight while maintaining stiffness, strength etc
2.Maintain weight while improving stiffness,strength etc
3.Drop weight and improving stiffness, strength etc