Most (all?) of the nicer mods are pretty much produced by hand in a small shop with one or two guys doing all the work. They aren't mass produced in a factory.
And there is enough demand that the guys making them simply can't keep up. The Provari, Darwin, Buzz Pro, Reo, and many more all have the same issue to one degree or another. As soon as they are made, they are sold, out the door.