At least some of the recent Clojure implementations are based on ClojureScript's compiler, which from what I understand is relatively moduler: http://blog.fogus.me/2012/03/27/compiling-clojure-to-javascr...

Lisps in general are appealing to implement. Clojure's core language is about as small as Scheme's, and there are a lot of Scheme implementations too. The parts of Clojure that set it apart from other Lisps (STM, persistent data structures) aren't always ported. I don't think any of the recent approaches to targeting C (Clj->Scheme->C, Clj->C) include STM or persistent data structures. I'd be happy to be pleasantly surprised though.

persistent data structures are implemented in ClojureScript. So they don't need to be ported for new compilation targets of ClojureScript like C in clojurec, I think.

ClojureScript's core library has a dependency on Google Closure, and are optimized with knowledge of javascript vms. david nolen said that v8, for example, is blazingly fast at sequential instructions so they take great care to avoid function calls in critical paths.

here's some discussion that alludes to performance concerns in js vms: http://dev.clojure.org/jira/browse/CLJS-284

here's the original commit for PersistentVector, note how low level the code is: https://github.com/clojure/clojurescript/commit/e615f4cd326e...

here's more discusson by david nolen: https://groups.google.com/forum/?fromgroups#!searchin/clojur...

more discussion about allowing low level code access to js-specific special forms for numerics: https://groups.google.com/forum/?fromgroups#!searchin/clojur...

Clojure was specifically designed to have multiple target implementations. That's the why it's approachable. As for why it's desirable, clojure is a high-level language and as such has certain niceties that are difficult to give up when forced to drop down to a lower level.

its clean and simple. the ClojureScript compiler is 2000 lines of clojure.