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.