I should think this has something to do with the high esteem in which both Lincoln and Kennedy are held. Lincoln is widely considered by historians to have been, if not our greatest president, certainly among the greatest. He led the nation through, arguably, its greatest test, and was killed nearly on the eve of victory. Kennedy remains highly popular in the public mind, though he is not rated among the best by professional historians. This may have something to do with the image of youth and glamour he projected, and a popular desire to imagine what might have been.

By contrast, McKinley and Garfield were, by all accounts, mediocre presidents in an age of presidential ineptness. You would be hard pressed to point to any other period in American history that produced as many middling-to-bad presidents as the late 19th century. The two decades before the American Civil War could perhaps rival it, but it would be a close run thing. In Garfield's case in particular, he died 200 days after assuming office and never really had the chance to demonstrate his talent, if he possessed any.