Richard Stallman wanted a free and open source Symbolics Genera, the LISP machine operating system that was developed jointly by MIT and Symbolics, until Symbolics found out that Texas Instruments had been shipping the same freely available source code on their LISP machines.
However, due to UNIX' and C's popularity around the time, real-world hardware (aside the few symbolic computer architectures) had stagnated to the sophistication level of the PDPs C had initially targeted, for this allowed C to remain 'portable', as in in its original definition of being easily ported to different computer architectures.
Symbolics Genera relied fully on the symbolic hardware architecture of the lisp machines that had features such as typed (type-safe) memory, user-programmable microcode, hardware-backed garbage collection and hardware-based generic function dispatch. In addition, the operating system sported features such as the entire operating system being reprogrammable at runtime, a global debugger, structural editing for data structures (including program code), disk storage for operating system states (called worlds), the general user interface being a LISP REPL, a GUI toolkit perfectly suited for a dynamic programming environment, and an always-available (then called online) self-documenting documentation system.
So when Stallman had his favorite operating system pulled out from below his feet, the choices he had were:>write a free version of Genera for LISP machines>write a free version of Genera for general computers (effectively C machines)>write a free version of UNIX for general computers>and maybe hopefully make the above as good as Genera was
And so, to have any sort of reach, by computer install base and the system familiarity, a free UNIX it is. Toss in Hurd (a dynamic microkernel), the info documentation system and emacs from Genera, the quality standards of LISP systems, and a general public license.
And the story of that's how LISP saved UNIX.