The Lem editor[0] and LispWorks IDE's[1] are implemented in Common Lisp.
Still, the reason for choosing a language for whatever are always more social and path-dependent than technical (reason 1: initial developer of whatever really likes the language, reason 2: language is seen as hip within some crowd, reason 3 (later in the game): management feels language is safe). Technical reasons for choosing a language typically tend to be post-hoc rationalizations. (I mean, no sane person would choose Javascript for an editor based on technical reasons alone, yet here we are.)
As much as I love Common Lisp, it's dead. It has 2 orders of magnitude fewer packages in quicklisp than Emacs has in MELPA - and Emacs is an editor, not a general-purpose programming language. SBCL has a handful of devs and moves very slowly - nothing else does at all. Maybe LispWorks, but that's expensive.
CL is also held together - and held back, hugely - by the standard that won't ever be updated. It's good to have it, but there are major omissions (code walkers, MOP) that won't ever be fixed.
As it is now, Elisp is more practical as a scripting language than CL. The gap will only continue to grow. Right now, CL has an edge in parallelism - native threads and mutexes (with all the problems they entail) work there, while with Emacs, the only parallelism you can get is through separate processes. On the other hand, async/await-style concurrency works quite well in Emacs, while in CL you're basically a few macros away from raw promises, and the only real asynchrony is through a pool of threads executing callbacks, and it doesn't play well with conditions and restarts and some other advanced features (notably absent from Elisp).
I love CL, but right now it's aged considerably, lost many of its unique advantages, and has little chance of ever catching up. It's a shame, but using CL in 2026 is not a superpower anymore - it's just one of the similarly-valued propositions, competing with other dynamic languages, still providing a few unique advantages, but even those are being implemented in other languages fast.
Still, the reason for choosing a language for whatever are always more social and path-dependent than technical (reason 1: initial developer of whatever really likes the language, reason 2: language is seen as hip within some crowd, reason 3 (later in the game): management feels language is safe). Technical reasons for choosing a language typically tend to be post-hoc rationalizations. (I mean, no sane person would choose Javascript for an editor based on technical reasons alone, yet here we are.)
[0] https://lem-project.github.io/ [1] https://www.lispworks.com/products/lispworks.html