People want to find out what happens to the characters, and want to keep reading, and turning the pages.
I've written enough books with real celebrities, such as Walter Payton and Hank Aaron and Billy Graham, to know that fame looks good only to people who don't have it.
Left Behind takes what to some people may be unbelievable predictions from the Bible and shows how they might play out. It makes the events of biblical prophecy understandable and thus believable.
Wherever you have people, you have factions and disagreements and personality conflicts.
It's made me more expectant of the imminent return of Jesus, and also more sensitive to the people around me. Knowing Jesus will return soon makes me want all the more to tell people about him and all that he offers.
People are scared to death and they're looking for something beyond themselves.