I mean, yes, it happened under an Ottoman sultan, but he was little more than a figurehead by that time (though there had already been anti-Armenian pogroms back in the late nineteenth century when the sultans did still have power). The Ottoman Empire during WW1 was effectively a triumvirate under Enver Pasha, Talat Pasha and Djemal Pasha, the three leaders of the CUP (Committee for Unity and Progress, I think that is, but I could be wrong - I'm not even entirely sure if the acronym is English or Turkish). They are the ones who are responsible for the genocide.

I don't know what the explanation is, but the Turkish reactions to the accusations of genocide are too emotional and furious to be merely related to practical matters, I think. For some reason, even though to the best of my knowledge the CUP triumvirate plays little or no role in the propaganda and national myths of the Turkish republic, they consider the accusations of genocide an insult to the republic.