It's because it's a LISP with good ecosystem. LISP is already strongly, but dynamically, typed from what I'm told. So, you get quite a bit of benefit. You could probably do the same in a statically- or gradually-typed LISP. Main difference is you'll catch a few more interface errors without tests. Also, if using specs or types, one can do automated generation of testing, enabled better static analysis, and/or improve optimization in compilers. Basically, the tools know a bit more about what you're trying to do.