Why does lincoln say that " we cannot we cannot consecrate.. we cannot hallow this ground," when that is the supposed purpose of the event? a. because those things had already been done by those who fought at gettysburg b. because not enough people attended the event to do this c. because the dead soldiers were the only ones who could accomplish this d. because only god could do it

C is your answer.
Because   People who have given their lives in a noble cause have done more than any living person can do because they have made the ultimate sacrifice. The real value comes not through labels and application of terms but through action.The bravery and sacrifice of the soldiers who fought here (in this case, Gettysburg) has done more to make this cemetery a special place than we can do by mere words alone. 
Hope this has helped