For the same reason as there are many algol-like languages.
The production-ready lisps are:
- Common Lisp: this one is standardized and the ecosystem is NOT fractured. Many implementations exist and work in parallel: SBCL, CCL, LispWorks, ECL, ABCL…
- Clojure (though there is also ABCL and I keep hearing good things about LispWork's Java interface)
- Schemes: some coming from university, with a fractured ecosystem.
Of course, nobody uses Arc for serious stuff (but we can have its syntax in CL and probably in Racket too).
The production-ready lisps are:
- Common Lisp: this one is standardized and the ecosystem is NOT fractured. Many implementations exist and work in parallel: SBCL, CCL, LispWorks, ECL, ABCL…
- Clojure (though there is also ABCL and I keep hearing good things about LispWork's Java interface)
- Schemes: some coming from university, with a fractured ecosystem.
Of course, nobody uses Arc for serious stuff (but we can have its syntax in CL and probably in Racket too).