Why is it flawed? People have to try and appeal to people with different backgrounds and living circumstances. That is a good thing. FDR was able to do it, as you reminded us....
They have to try to appeal to people in Michigan, Wisconsin, Pennsylvania, Georgia, Arizona, and Nevada. That's it.
With a National Popular vote, the candidates could go stump in Oregon, or Florida, or any of the 50 states, because everyone of those states, and every county in every one of those states, has persuadable voters, from a far larger set of backgrounds and living circumstances than those six states.