Because of three basic reasons. This is not only for the Armenian Genocide. This is generally for all genocide and all mass killings. One, to respect the victims, to accept their dignity, and to give an end to their traumas.
Second, it is very important for the reconciliation in a society, for the democracy, for the human rights. If a society cannot face its own history, it cannot establish a democratic future.
And the third factor is related to the second one. If you want to say the sentence, "Never again," it can only be possible if society faces its history. If a society, if a state doesn't acknowledge its wrongdoing in the past, this means there is a potential there, always, that it can do it again.