Part of the problem is how physically big the US is compared to most countries. Popular vote only would mean that cities (even a small set of them) would hold such a large percentage of the vote that the farmers and workers who drive the engines of agriculture and manufacturing could have almost no voice. Additionally, when this was all created, each state just wanted to govern themselves and they needed a compromise for what the federal government would be. So, instead each state was (and is) given a relatively proportional number of "special" votes to cast for President based on population. These have caps though so small, rural states get a little more power than they otherwise would (2 for Idaho instead of probably 0 e.g.) and super large ones get a little less. (This is not that dissimilar to how our Congress is set up). Additionally, each state is given the ability to decide how to vote for president. A state could say that the Governor just picks one or have an in-state vote that is a popular vote within that state (how most work) or anything they want.