A real compiler backend takes a bit more than a weekend project, and it is hard to know the real reasons, technical or personal life of the coders, for the current state of the projects.
I think the real problem with the native Clojure compilers is that Clojure is designed to be symbiotic with host platforms that offer a much higher level of abstraction. For example, to perform well, all those persistent data structures require an advanced garbage collector. The JVM, the CLR, and the leading JavaScript implementations offer this; it would take a lot of work to implement one in the absence of one of these mature platforms.
A real compiler backend takes a bit more than a weekend project, and it is hard to know the real reasons, technical or personal life of the coders, for the current state of the projects.