Because we effectively live in a Unix monoculture world, in terms of operating systems that you can actually use and study the inner workings of.
It was a legal anomaly that resulted in early Unix (aka Research Unix) being distributed with its source code for free to universities, which was enough to get the ball rolling on stuff like BSD and Lion's annotated V6 source code, that by the time AT&T decided that closed source commercial Unix was the game it wanted to play, the cat was already out of the bag.
By the time the free software and open source movements had gained some ground, enough people had studied or worked on some kind of Unix kernel and Userland source code that projects like Linux, Minix, and Free/Net/Open BSD were feasible. The fact that Linux running on x86 subsequently ate the world was probably something few people saw coming.
The other lineages of operating systems, e.g. Windows NT, OpenVMS, IBM's various offerings, either never have their source released or only have their source released long after they're obsolete.
It was a legal anomaly that resulted in early Unix (aka Research Unix) being distributed with its source code for free to universities, which was enough to get the ball rolling on stuff like BSD and Lion's annotated V6 source code, that by the time AT&T decided that closed source commercial Unix was the game it wanted to play, the cat was already out of the bag.
By the time the free software and open source movements had gained some ground, enough people had studied or worked on some kind of Unix kernel and Userland source code that projects like Linux, Minix, and Free/Net/Open BSD were feasible. The fact that Linux running on x86 subsequently ate the world was probably something few people saw coming.
The other lineages of operating systems, e.g. Windows NT, OpenVMS, IBM's various offerings, either never have their source released or only have their source released long after they're obsolete.