It is rarely because they have stopped caring. More often the class they had at ten no longer fits at fourteen. They are reading alongside much younger children, the lessons feel like a repeat of something they did years ago, and the timetable clashes with homework, sport and friends. Eventually it gets dropped and nobody quite decides to stop.
Teenagers also tend to be more self-conscious about their reading than younger children. A fourteen-year-old who never quite mastered the basics will often avoid reading aloud at all rather than be heard making beginner mistakes.
Pushing harder rarely fixes this. Lectures about why the Quran matters tend to land badly at fifteen. What works better is a class that respects their time, gives them visible progress within a few weeks, and puts them in charge of some of the decisions.