Matthew Dickinson ponders it:
Why is it so difficult to defeat an incumbent president in the modern era? One likely reason is that the office is much more visible, so that presidents simply by virtue of carrying out their duties in a non-partisan way, such as providing disaster relief, can score political points. It may also be the case that in an era of nuclear weapons and other WMD’s, the presidents’ foreign policy role enhances their political standing. That is, as national security issues loom larger in voters’ calculations, the incumbent president’s foreign policy role is magnified. Moreover, despite the criticisms his comments entailed, Romney was right when noted – albeit perhaps not in the most diplomatic manner – that Presidents are relatively well situated to influence policies in ways that reward key voting blocs. All this is somewhat speculative, of course, but I am persuaded, in the absence of countervailing evidence, that modern incumbents generally run for reelection with advantages that their premodern forebears did not possess.