Batman is more popular because he appeals more to baser human nature. He's rich, good looking, he's a genius (even if the movies have never portrayed that in anything approaching an effective form), he drives an incredible car, women want him, men want to be him, he's motivated by frustration, anger and a desire for revenge. He beats up people he has no business even looking funny at and no one can tell him what to do. He wears black and, at least recently, is played 100% as a straight tough guy with no room for feels. He's a CEO and gets his powers from buying things. He's the modern epitome of "manliness" in corporate-owned consumerist America.
Superman, on the other hand, has for years been meant to appeal to higher ideals in people. He's strong, sure, but that strength has been dedicated to protecting the weak. He's kind, optimistic, humanist, forward-thinking, community-minded. He was the original Social Justice Warrior in a much more literal way. He gets his powers, in a metaphorical sense from embracing the dream of his birth parents that he could have a full, happy life away from the doomed Old World and all of the intensely calcified, negative, apathetic thinking it represents. Superman is directly powered by hope, represented in this case by a young sun with millions of years yet to give. He's the good stuff in people many of us just don't find the time or do not bother to be and he's unironically American. He's a borderline socialist symbol of the power of good-natured common men coming together to make something great, not with money and brains, but with care, their own two hands and their whole heart.
You have to be an idealist to buy into one of those, for the other you can be anybody. And that's why Batman will always be more popular.
But, as an idealist, I will always see Superman as the superior hero. He's not only a better, stronger person, but a better, stronger story. People become the stories they tell about themselves. We can remain past-haunted and bitter, clutching our wealth as we fight a futile and doomed battle with our inner demons and the world. As someone living in 21st century America, that certainly sounds like what we've been doing so far. We can keep doing that, no one is stopping us.
Or we can go join Superman in the sunlight.
You don't have to decide now. He'll wait. Forever and a day if he has to. He can do that. He's Superman. We're the ones that won't last forever.