All of the data centers within a region are in the same physical location. Aether, Primal, Crystal, and Dynamis are all within a few thousand feet of each other. That's the primary physical limitation.

The primary limitation beyond that is Japanese players don't want to play with people who don't speak Japanese, and some of them don't want to play with non Japanese people regardless of whether or not there's a language barrier. Picture the American redneck screaming "speak English" and multiply that by ten.