Because all Orcs are barbarians, of course.
That's not what I'm getting at at all.
What I'm saying is that certain techniques probably have to be taught and are very unlikely to be developed independently. Some forms of wizardry the Orcs and all peoples with a Shaman/Barbarian history probably use are very common like alchemy, some forms of restoration, destruction and mysticism. But the powerful teleportation network maintained by the Mages Guild probably requires multiple resources and fully trained teleportionists.
However I did read that there IS a Mages Guild in Orsinium. In "The Black Arts On Trial" it is revealed that Orsinium has an Arch Mage, who had to be replaced because the old one was a Necromancer. With the presence of a Guild it's possible there has been some cross training with Cyrodill, Morrowind and other provinces and therefore there are probably more than a couple of Orc Mystics who can teleport.