I never hear that the West is ever talking about Nanjing Massacre.
This is as true as Armenian Genocide.
Why is the international community so vocal about the Armenian genocide?
So while both are horrific events, from the perspective of history one was a long, slow, systematic prosecution by a country of its own citizens, while the other was a short, intense burst of violence by an uncontrolled army against the enemy. The Nanking Massacre would be far more headline-grabbing as it happened, but the Armenian Genocide was far bigger overall.
One obvious reason why the international community is particularly vocal about the Armenian genocide right now is that it started on April 24th 1915, i.e. one hundred years ago almost to the day. There is almost always heightened media interest in historical events when their centenary or other round-number anniversary comes round.