Yeah, it of course would make sense to support the newest LTS, however that would mean bumping Yocto version to something like 1.6 at least, which is a significant change (kernel version, package recipes), which would require porting and then extensive testing - so if they go for that (I have no idea what the plans are) I can see why it may take some time
Plus 12.04 is still a supported one (till ~2017), so not that bad either