Is that really true? From what I've seen from paleoanthropology, the lumpers versus splitters war still rages fiercely - see Homo neanderthalensis or H. sapiens neanderthalensis.
Though I would argue that at least in the modern times, communication and transport mean that no identified group of humans is actually totally isolated anymore from the other humans, which makes it more difficult to argue that they can be different species on this ground.