Stallman also chose to build Emacs and Emacs Lisp. He could have made an editor extendable in c only, but did not. Emacs' problem, then and now, was non-existent, then terrible DOS and Windows support.
Really, the problem was that there was no DOS or Windows lisp available for cheap, or free. Even GCC was only available thanks to a small company, Delorie, who made djgpp.
I would have greatly preferred to program in lisp; but on DOS and Windows all I had was C, Pascal, and Basic; thanks to Borland, Delorie and Microsoft. Later I had Java, Python and Perl; again thanks to companies like ActiveState.
The focus on UNIX-only kept these other languages, free or not, from being useful to the broader public.
> Emacs' problem, then and now, was non-existent, then terrible DOS and Windows support.
I don't know when GNU Emacs first became available under DOS. If you look at the GNU Emacs 18.59 source distribution (October 1992), while it doesn't support DOS/Windows, its FAQ mentions a DJGPP-based port to 32-bit machines called Demacs (which wasn't pure GNU Emacs, it was actually based on the Nemacs fork which added improved Japanese language support).
The many limitations of MS-DOS (8.3 file names, no multitasking) meant that it was essentially impossible for GNU Emacs under MS-DOS to work as well as it did on more capable platforms. Windows 3.x/9x/Me laboured under many of the same limitations – 9x/Me didn't have a 32-bit command line environment, all it had was DOS boxes and COMMAND.COM, plus some horrible kludge by which 32-bit console applications would have their I/O routed through a DOS process (CONAGENT.EXE) via a VxD.
While NT-based Windows fixes many of those problems, it doesn't fix all of them – for example, until relatively recently (some Windows 10 build), Windows had no pseudoterminals (except for various flaky unofficial workarounds), which put big limitations on Emacs support for subprocesses compared to other platforms. Now at last it does, but I'm not sure if GNU Emacs has been updated to support them.
IIRC, Stallman's original mission was to write a 'free' operating system (-> GNU Hurd). For that he needed a C compiler, an editor, a Lisp, etc.
Integrating GNU Emacs into proprietary operating systems like Microsoft DOS / Windows or Apple's MacOS wasn't his priority.
There were a bunch of other Lisps on DOS/Windows: Xlisp, MuLisp, CLISP, EcoLisp, RefLisp, LinkLisp, Corman Lisp, Golden Common Lisp, Procyon Common Lisp, Allegro CL, Medley, LispWorks (later), NanoLisp, Software Engineer, Star Sapphire Common Lisp, ...
Maybe they were too late, didn't fit your requirements, etc. But it was not that there was none.
Really, the problem was that there was no DOS or Windows lisp available for cheap, or free. Even GCC was only available thanks to a small company, Delorie, who made djgpp.
I would have greatly preferred to program in lisp; but on DOS and Windows all I had was C, Pascal, and Basic; thanks to Borland, Delorie and Microsoft. Later I had Java, Python and Perl; again thanks to companies like ActiveState.
The focus on UNIX-only kept these other languages, free or not, from being useful to the broader public.