Defenders of the national popular vote winner claim that America is the only nation with an electoral college. The term "electoral college" is likely uniquely American, so sure, we're the only ones with one. The U.S. isn't the only nation that doesn't determine its head of state by who got the most popular votes. Many nations have a parliamentary system. Their prime minister is determined by an election in one of their houses of government. The people who elect the prime minister are elected by voters in regional districts. This'd be roughly the equivalent of the Speaker of the House being the head of the government in the U.S. In 2012, Democrats won the House of Representatives vote and yet the Republicans had the majority in the House. If the U.S. were the U.K. Paul Ryan would've been President. In 2016, Republicans won the House of Representatives vote. If the U.S. were the U.K. Paul Ryan would be President. The electoral college is pretty much an extension of using voting districts to determine who gets elected. And Donald Trump did win a majority of the House of Representatives districts.
The founding fathers didn't want the winner of the national popular vote to with the Presidency. For the first several Presidential elections most states didn't even have a popular vote. The first election where every state had their citizens vote for President was in 1864. Electors have only been required to adhere to the popular vote in their state in the years since. I'm sure there were delegate to the 1787 Constitutional Convention who wanted the people to choose a President but that's not how the Constitution is written.
This election crystalized why we need the electoral college. President Obama won the median state by 3.0%. Hillary Clinton lost the median state by 3.6%. Donald Trump had a better margin than Mitt Romney in 39 states, while Clinton had a better margin than Obama in 11 states and D.C. The median margin change was 4.2%. Donald Trump did better than Mitt Romney in most places and in over half his margin was bigger than Barack Obama's overall margin of victory.
Hillary Clinton improved on Barack Obama's margin in three big states, California, Massachusetts, and Texas. That's obscured all the places Donald Trump improved due to their sheer size. One thing these three states have in common is that neither candidate was trying to win their votes. Neither campaigned in any of them, spent any money, or mobilized volunteers. Trump did better in the states where the two were actually trying to win votes.
What this shows to me is that if we elected the President by national popular vote the candidates would concentrate their efforts in a few urban areas to the detriment of everyone else. With the electoral system candidates need to compete in diverse group of states that have different needs. Issues that matter in the Rust Belt might not matter in the Sun Belt. Yet a candidate needs to appeal to both. If a candidate concentrates solely on the big cities in a few states there's not much of diversity of interests. Sure, there are some states that don't matter in the current system because they are in the bag for one of the parties but at least more states matter than would matter with a national popular vote.