For three reasons. The first is that he was shot on Good Friday and died on the morning of Holy Saturday. Americans of 1865 were much more religiously observant than they are today, and the coincidence of the events with Holy Week was too much for them to merely pass over. Second, John Wilkes Booth killed Lincoln when he was in his prime, not physically, but as a moral force around which Americans could cohere. And the third reason that the Lincoln assassination remains with us is the charitable, open-hearted spirit that Lincoln held toward the defeated South. Those Southerners who thought Lincoln was their great foe were about to learn that he was MUCH better than the man who succeeded him in that respect.