Variety of reasons
1. Use and development of software which is not license compatible with the Linux kernel.
2. Smaller footprint
3. While Linux documentation and code has improved over the years it is still not the right OS to teach people. If you want to teach people, BSD is a better choice as it is "written by the book" and documented properly. End of the day the primary goal of RPi is education. If it supports BSD this means its scope is no longer just kids and Scratch - it is a viable tool in a University classroom in an OS design course (if there are any universities left which try to teach things properly instead of asking the industry if it likes it with ice cubes or with hot coffee)..