2. FDR was a tough act to follow.
Let's face it. Following a four-term president who overcame polio to lead America through its greatest economic calamity and to victory in its greatest war sets a pretty high bar. None of his successors would measure up. Truman, who likely had the third-hardest job in American politics (following FDR -- No. 3 after John Adams, who followed Washington, and Andrew Johnson, who succeeded Lincoln), did pretty well, particularly in fashioning a wise set of Cold War policies. But the idea that any president would best Roosevelt in the achievement department died with him. That all of FDR's successors are still judged by his remarkable 100 Days is a testament to the reality that all would live in his shadow. In 1951, driven by Republicans and southern Democrats, the 22nd Amendment was ratified, restricting a president's tenure to two terms. FDR's enemies thought they were getting even, but what they really did in ensuring that there would be no more FDRs was to elevate the real one into presidential immortality.
3. America's modern crises make greatness very hard.
Greatness in the presidency is driven by severe crisis that encumbers the nation for a sustained period of time. Washington, Lincoln, and Roosevelt were America's greatest presidents because they faced and overcame the three greatest challenges in America's history: the birth and consolidation of the republic; civil war; and the Great Depression and World War II. The Founders wanted a strong executive, but one who was accountable also, constrained by checks and balances as well as shared and separated powers. The American system moves only when it's shocked, and it allows a president to tame what is an inherently unruly political structure.
But crisis only opens the door. Unless a president also has the character and the capacity to know what to do and how to do it, they will fail. James Buchanan and Herbert Hoover weren't up to the challenges; Lincoln and FDR were. Since FDR, America hasn't had the kind of emergency that has offered the chance for both heroic crisis management and the fashioning of some transformational legacy that would change the country in a fundamental way.
America has had plenty of crises, to be sure, but none that have been inescapable, relentless, and nation-encumbering. In fact, America's modern challenges have become routinized. That's what happened with the Cold War, even with the 9/11 attacks. They became what presidential scholar Richard Neustadt called "inside government crises" rather than outside crises. Indeed, where presidents have had great moments -- John F. Kennedy over Cuba, Lyndon B. Johnson in the wake of JFK's death pushing transformative change on civil rights and social programs -- they resulted from real crisis or trauma. The 9/11 attacks might have been such a moment, but it was wasted by a president who took the country into discretionary war and away from using the tragedy to promote a sense of sustained national identity or purpose. Obama had his economic crisis -- but without Depression-era 25 percent unemployment and bread lines, it wasn't sufficient to force Republicans to cooperate or allow for dramatic presidential action.
America's contemporary challenges are severe, but they're slower bleeds. Debt, deficits, and decaying infrastructure produce delay and division -- not consensus. And problems of this nature don't afford much space for heroic presidential action. Balancing the budget isn't quite defeating Hitler, but the structural polarization in modern politics makes even the former very hard to achieve.