One of the benefits of Clojure is that it's Just Another JAR. If you want the to use a new version of the Java language, you have to convince your devops people to deploy a new version of the Java runtime, which may take time. If you want to use the latest version of Clojure, all you have to do is change a dependency in your build, and you can use the latest version of Clojure with Java 6 if need be.
Maybe eventually compatibility with Java 6 will be abandoned, maybe eventually they'll require 8, but the Clojure devs really don't want to do that lightly.
Clojure has a fantastic track record of not breaking existing code. Changing clojure's `#inst` to a different class would probably entail such a backward-incompatible change. There are also still people running Java 1.7.
The argument put forward here (and elsewhere) seems to be that
we want Clojure to gain the advantages of being on the bootstrap classloader in Java 9 while still targeting Java 7
That's pretty unconvincing to me. If you're shipping jars that are intentionally compiled against old JREs then you might not get the benefits of new JVM features. Them's the breaks.
Just curious, why haven't they switched to java.time.LocalDateTime? Java 8 has been out for 3 years now.