I wouldn't say *nobody* uses it, even though it's certainly much less popular than ANSI Common Lisp.
If anyone is curious, the spec ( https://islisp-dev.github.io/ISLispHyperDraft/islisp-v23.html ) is quite short. As far as full implementations go, there is the commercial ISLisp ( http://eligis.com ) and the moderately-active open-source Easy-ISLisp ( https://github.com/sasagawa888/eisl ). There are also several subsets, but I never tried them.
It may be down to personal taste, but I just like the language for hobby programming. I like that it has few surprises, being very similar to CL & ELisp so you can port snippets across; but on the other hand is much smaller than CL.