Hi!
Thats because it isn't part of the skincluster, you probably didn't have it selected when you created it.
You can add joints to the skincluster by selecting the joint and using the Skin->Edit Influences -> Add Influence command.
But, I would argue, that this joint isn't really needed. While Humans technically have an upper Jaw bone, it is fused together with the rest of the Skull and can't move on its own. So most rigs only have a lower jaw joint and a head joint anyway.
Also, I'm sorry to say, but I only had your rig open for about 5 minutes and I already saw quite a few problems with it. You might want to do some more rig testing and fixing before moving on to the skinning stage.
I hope it helps!