A couple of years ago, RISC OS was dropped from the Raspberry Pi Foundation’s NOOBS (New Out of Box Software) system, designed to make it easy for new Pi users to set their system up with whichever available operating system they wished. Our beloved OS was dropped because it took longer than expected (and hoped) to support the Raspberry Pi 3.
However, RISC OS is now back in the system, so new users (or even more experienced ones) can now choose RISC OS when they first set up their Pi using the Foundation’s tools. For more details, see the RISC OS Open Ltd announcement.