The problem being that doing it by popular initiative is probably unconstitutional. The manner of selection is given solely to the State legislatures.
Article 2, Sec 1: "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors..."
Article 2, Sec 1: "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors..."
Good point. I really don't know how that's been (or is being) addressed.