First of all, almost all Scientology scriptures are public. If we say "confidential material" this refers to less than 1%, or some dozen pages out of thousands. Those are confidential in a sense that only Scientologists who reached a certain grade (called OT level) can study them.
Confidentiality of certain religious scriptures is not unique to just Scientology. Many other religions do the same. Judaism, Christianity, Tantric Buddhism, and gurus and swamis in India all have kept certain scriptures confidential. Rev. Dean Kelley of the National Council of Churches cited biblical injunctions that advanced spiritual knowledge be kept from adherents who are not ready for it.
Professor Darroll Bryant of the University of Waterloo said:
“The distinction between ‘esoteric knowledge’ available only to initiates and ‘exoteric knowledge’ available to all has long been part of the religious life of mankind.”
So, confidentiality regarding certain scriptures has been practiced by religions all over the world for a very, very long time.
Darroll Bryant Findings