The defining characteristic of teenagers is independence/control seeking. They often reject their parents' oversight, worldview, and life advice in order to seek out their own path into adulthood. The idea that everything their parents think is wrong and that they hold a deeper understanding of the world is appealing. That takes many forms. For me, it was exploring atheism and art.
In witchcraft, not only do they get that, but also get an apparent means to assert control over the world that they are seeking through various forms (tarot, ouija, and other forms of spellwork and divination). On top of that, it specifically appeals to women through ideas of naturalism, sisterhood, and femininity and has a long tradition of women rejecting the norms of society and being entirely independent. I can see the appeal.