Why, in Lincoln's view, is it not possible for the crowd to consecrate or hallow the cemetery?

This line is an excerpt from the speech Lincoln gave on November 19, 1863 during the American Civil War, at the dedication of the Soldier's National Cemetery in Gettysburg, Pennsylvania.

Lincoln argues that "the brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract."

Lincoln is implying that the blood of the victims has already made the soil sacred, and that it would be presumptuous of us to think that we can consecrate (make sacred, hallow) it any more than they have.

Answer: The Ground had already been hallowed by the sacrifice of the dead.