Why is it difficult to tell from fossil and archaeological evidence alone whether humans are the same species as the Neanderthals?

Consider the following: all sorts of dogs that are descendant from the grey wolf are one single species, yet their morphologies vary in the extreme. Neanderthals were morphologically very close to the anatomically modern human, so it's difficult to tell, based on morphology alone, whether they're a different subspecies or an actual separate species.