Why might Lincoln refer the audience to think about that particular year when they are burying soldiers who fought in the Civil War?

Lincoln is referring to this year, to connect with the audience, and help them to remember the beginning of this country. At the time, the civil war and the countless number of deaths were immense. He wanted the Civil War to be over, and when referring to “a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal” he is saying that the war should not be happening, the deaths should not be happening, and that we need to return to the nation of unity that we once were.

hope this helps :)