'Tis a good question that I, in mild form, have aksed a few times myself. I have to admit that anyone playing TSoG from scratch would be confused as to why you can't visit Rubat. I guess this is all to do with the way TSoG has been developing, including interruptions for TPA2 and adding in different features at different times. Seems like the area isn't critical at the moment, so I guess we'll probably get Rahel saying something along the lines of "we can't go there" or the Rubat Chieftan refusing to help 'cause of Rahel, who knows?
I do wonder though when that area will be completed? (Seems it's in the same category as Somnus or the Deeper Downs.)