Most of the research that Typed Clojure is based on didn't even exist when Clojure was created. Rich just made the best language he could, and waited for type theory to catch up, as it were!

See this goldmine of papers for Typed Racket http://www.ccs.neu.edu/home/samth/

Clojure is a small core of a language that can be flexibly extended with libraries in ways that other languages would require built-in support for. See core.async for another example of this.

Oh I see, I didn't know that. Thanks.