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.)

[0] https://lem-project.github.io/ [1] https://www.lispworks.com/products/lispworks.html