In the Gettysburg Address, why did Lincoln leave out references to the two armies by name and to slavery?

I would argue that Lincoln did this because he was trying
to be conciliatory.  He was already looking forward to a time when the war would end and
the United States wouild be a single country once
again.

President Lincoln's main goal in fighting the Civil
War was to preserve the Union.  He believed that it was that important to keep the
United States together.  In the Gettysburg Address, Lincoln was trying to promote this
agenda.  If he had referred to the separate armies and if he had referred to slavery, he
would have been talking about the things that divided the country.  Instead, he wanted
to talk about the common heritage and common ideals held by all Americans.  This is why
he did not mention the separate armies and did not mention slavery during this
speech.