The original lab that invented Unix at AT&T has officially been shut down. This very team started it all: FreeBSD, OpenBSD, HP HP-UX, Compaq Tru64, Linux (RedHat, Debian, Slackware, Gentoo… etc), SCO Unix, IBM AIX, Sun Microsystems Solaris, SGI IRIX and then of course, Mac OSX, all of which derived from the original Unix conceived at the Bell Labs.
And then there is Microsoft, DOS and then Windows (3.1 through XP), the legacy Mac OS, and let’s not forget OS/2. There are others that never made it to the mainstream, but who cares.
After looking at this large list of Unix operating systems v.s. the rest, it’s a no brainer that Apple decided to go with a Unix-based strategy to capitalize on its maturity and stability.