There are two answers:
A) You answered your own question.
B) If they want to write Jewish books. While there is somewhat of a mainstream market for Jewish books, they’re not necessarily publishable if they’re al taharas hakodesh, if that’s what you want your book to be. (Maybe, but probably not. I remember reading that the author of Go, My Son considered publishing in the mainstream press but did not for that exact reason.) Unless you publish at a Jewish publishing house that distributes in the mainstream marked or self-publish, this is likely to happen.