The reason is imho mainly due to how institutions evolve (slowly). Computing and the arts is currently experiencing a primal boom, which you can see in a number of departments, festivals and prizes dedicated to that are popping up everywhere. It won't take long before most places offer this kind of pathway. Also, for various reasons, I feel like having a CS background and coming to the arts has been an easier route than the other way around, which might have had consequences on which departments started offering this kind of training, and how computational arts have been received in the broader arts community.
I am personally interested in the writing/literature side of all this, and indeed haven't yet come across a department of creative writing that would have a focus on computation (SFPC might be the only place that at least nominally places poetry at the core of their endeavours, even if the activities of students and teachers, so far as I know, are really exploring all art forms).
http://fweet.org/pages/fw_srch.php