Book publishers, like any other business, are responsive to what customers are buying and what they aren't. Publishers lose money printing books that no one reads, so the most successful publishers study sales trends and select the manuscripts that are most likely to sell. Young adult literature is a large market, and publishers adapt to what teenagers are reading to figure out what genre and story trends are popular at the moment.
The film industry is always looking to make new movies, and looking for inspiration in existing books aimed at teenagers can save a great deal of creative effort on the part of producers and movies. Executives can pick up a book and immediately assign some screenwriters to adapt it for the big screen, as well as hire lawyers to appropriate a deal with the author and publisher. They can also use the trends that are currently popular in teen literature to pick which scripts to turn into movies.
When the film industry makes a movie based on or inspired by what is currently possible in teen movies, the industry is in no small part targeting the teenage demographic to be a prime source of the project's profits. Teenagers spend a lot of money. They are not only what All Business refers to as "enthusiastic customers;" they can also drive money toward the movie's profit margins by convincing other teens to see the movie, magnifying advertising efforts at no additional charge and influencing their parents' spending behaviors.
Making movies based on what teenagers are reading gives film executives the chance to get detailed market research on the performance and popularity of the product before they spend a dime making it. Using market data from the sales and popularity of teenage literature can help predict when the time is right to make a particular movie, a genre of movie or extending a franchise into another sequel.