I don't know why this isn't available, maybe it is and I'm overlooking something. Overall though, it's a great disservice to our teams that we cannot create custom statuses for subgroups. The alternatives are applying the statuses they need to everyone as a whole (which is messy and confusing) or creating an entirely separate group (which would seriously screw with our reports).
This already has an idea in the idea exchange! Give it a vote please!