Popularity.

Lincoln, a very famous president, is remembered as being a great leader. His image is everywhere in the United States and the civil war is taught pretty uniformly throughout the United States.

JFK is still in living memory for most people and JFK was a very charismatic president.

By contrast McKinley and Garfield were popular at the time, but have the misfortune of being gilded age presidents, which were pretty weak and didn't do a whole lot. Congress was the more powerful force during the period and the government was firmly committed to Laissaze Faire (spelling) (An oversimplification but this is how it is taught in schools). To give you an idea, Garfield was president for 6 months and the "greatest" thing McKinley did was die so that TR could take the presidency(in popular memory).

Sources:

The Bully Pulpit by Doris Kearns Goodwin Destiny of the Republic (about the Garfield Assassination) Lincoln by David Herbert Donald

I think its to soon to soon to really judge the historical impact and remembrance of Kennedy's assassination, especially in the light of the revelations of his private life. Will it have the same weight in a hundred years? In my opinion no. Lincolns however coming as it did at the the end of the civil war after his role in persecuting the war and maintaining the union i believe will.