For a more OOC sort of explanation...
CNR placement has as much to do with where things are according to world lore as it has to do with the greater issue of world balance.
Putting mid-to-high level CNR within just a few areas of an applicable crafting center makes them too "easy", even if the CR of the cave was equivalent to the current placement(s) of such things on Central. But then that creates other problems, as we would have a pocket of mid-to-high challenges surrounded by low-level challenges, which in itself is not terribly appropriate.
Part of crafting balance is the rate at which someone can craft a given product. The higher level of item that is crafted, the more CNR that's required (in general) and the farther away one must go to get it.