The most puzzling thing about the Great Tusk is that they ever went extinct at all. Humans, for all their destructive power, could never have produced such an outcome; even their weaker modern descendants dominated human wars for generations, and a rolling Donphan simply can not hope to equal the force which paleontologists ascribe to a charging Great Tusk. Yet Great Tusk postdate the other mass extinctions, and appear to have died out in the Miocene – despite nothing in the fossil record of that time appearing at all capable of confronting a fully grown Great Tusk, and unarmored juveniles, akin to modern Phanpy, appearing to be a Pleistocene development.
Paleontology must turn to astronomy to answer this question, for traces of an Ultra Wormhole incident are detectable at precisely the point in time when Great Tusk are replaced with less impressive pachyderms. We do not know why these aliens so valued this pokemon's ivory, but the pattern is undeniable and has modern comparanda; before boneyards became valued for their insight into natural history, many tusks were severed by humans to be repurposed into a variety of artworks, tools, and instruments.
The prospect of an alien moon full of Great Tusk statues has fascinated astronomers since the temporal correlation was first noticed. But artwork on Earth is lucky to survive a couple thousand years, so it would be truly remarkable, even accounting for relativistic effects, for any tusk-based works to survive 14.7 million. In all likelihood, the only thing which remains of this first contact is the modern, medium-tusked Donphan.
One may fairly describe this as a tragedy, but if the Scarlet Book (an admittedly problematic source) is to be believed, then one should not forget that the only recorded encounter between Great Tusk and humans was fatal to the latter.