Read The Federalist Papers, 68 in particular.
This end will be answered by committing the right of making it, not to any preestablished body, but to men chosen by the people for the special purpose, and at the particular conjuncture.The point being that the people (at the time, through their State legislatures) would select the electors in a manner that each state saw fit. So again, seeking to maximize their influence on the election, it seems almost tautological that eventually the electors would be chosen to vote for a particular candidate, and to maximize a state's influence, a single candidate should be chosen.
Sure, the Founders did not necessarily know exactly how it would work out. But they didn't have to. Instead they constructed a minimal framework that ensured fairness and separation of powers, while leaving as much freedom to the states as possible. Thus it is entirely within the Founders' general intent, I argue, for the states to decide to select electors they way they do now. It is entirely consistent with their desire to maximize the power of the states over the central government.