In the Usenet newsgroup alt.computer.security, in article
<GV0we.12200$xm1.5790@fe06.lga>, Noob wrote:

How long is "sometime"? SELinux was initially available in late 2000.

From their FAQ:

Why was Linux chosen as the base platform?

Linux was chosen as the platform for the work because of its growing
success and open development environment. Linux provides an excellent
opportunity to demonstrate that this functionality can be successful
in a mainstream operating system and, at the same time, contribute to
the security of a widely used system. A Linux platform also offers an
excellent opportunity for this work to receive the widest possible
review and perhaps provide the foundation for additional security
research by others.

So it might appear to be popularity.

Old guy