John Nichols (The Nation) tells us why. It boils down to one word in Article II of the Constitution: SHALL! Nichols documents several previous instances in which SCOTUS nominations went forward to confirmation in the final year of presidencies.
All the [Republican candidates] joined in this egregious assault on the basic premises of the Constitution and on the system of functional governance that is supposed to extend from it.
It fell to President Obama to show respect both for the memory of Justice Scalia—who he recognized as “one of the most consequential judges and thinkers to serve on the Supreme Court” and as a jurist who “dedicated his life to the cornerstone of our democracy: the rule of law”—and for the dictates of the Constitution. “I plan to fulfill my constitutional responsibilities to nominate a successor in due time,” said Obama. “There will be plenty of time for me to do so, and for the Senate to fulfill its responsibility to give that person a fair hearing and a timely vote.”
The president is not naive. He knows that his nominee must secure the approval of at least a handful of Republican senators in order to be approved. But President Obama has already nominated two justices who have been confirmed with bipartisan majorities. He has a history of striking the right balance.
The president was right to embrace the responsibilities of his office.
And the Republicans may well suffer a backlash should they arbitrarily deny Obama's nominee.