Imagine you're one of the 5.2 million registered Republicans in California - the largest and bluest state in the country. Why vote in the presidential election? Your voice has never counted. Not in a half a century. If you're within those borders, you suffer a tyranny of a majority you could never overcome, so you don't vote. Worse, republican candidates never campaign in your state, because it's a lost cause for them. It's a waste of resources. Instead, they focus on the states where results are less predictable. Michigan. Pennsylvania. Ohio - states where blue collar and white collar are more split.
After a few decades, you don't vote anymore. There no real reason for you to vote. You've never had a voice.
So in the most populous state in the union, you have an enormous swath of disenfranchised people who don't vote, have no reason to vote, and may have never had a reason to vote in their entire lifetimes. Now you understand why the popular vote doesn't matter.