I am not suggesting that efficiency and quickness of vote is a strong reason, but things would change a lot in a society where we demand immediate results.
Looks like that interstate compact is a work-around the constitution, so it is still electoral votes to decide the winner.
But you well know that its is politics that holds this up, not reason or common sense.
My main point is that a popular vote in a large country with non-homogeneous voting machines will make for a difficult process where it would be tough for a candidate to concede early.
And it would be best to do this by Constitutional amendment.