Some stories are popular beyond any expectation -- even stories that are no better-crafted than average, or have serious flaws. I notice that the more popular series speak to the current situation: they touch the average person's daily struggles. Consider the basic premise behind the most popular stories of the last decade:
Harry Potter: An unappreciated boy with a dreary life finds a world that is enchanted and exciting -- where he is welcome, and he matters.
Twilight: An only child from a broken home finds a large family where she belongs, and love that will last literally forever.
Hunger Games: the common people fight a system rigged against them by a rich, out-of-touch political class in a corrupt and decadent capitol.
The themes of the day are being lonely and unappreciated, wanting to find love that will actually last, and living in a big system that is rigged against the little people. The newest entry in this list, Hunger Games, has a heroine whose early victories include seeing the politically-assigned "enemies" as not so different than herself -- or her younger sister. In that world where hatred and conflict are promoted, an act of defiance can be as simple as holding hands, or honoring a little girl's death with some wildflowers, or giving someone a small loaf of bread.
My point here is seeing how simple it is to undermine a corrupt system. Every act of honesty, decency, or compassion is good not only for its own sake, but also as a threat to the surrounding darkness. These books speak to daily life because so many people are looking for a warm welcome, for a chance to matter, for a place to belong. And like the Hunger Games' heroine Katniss discovered: it is a powerful thing to realize that other people -- even "the enemies" -- really are just like you.