The reason we use a different extension is because a build.boot file is not semantically a Clojure file.

Clojure's default namespace is 'user'; .boot files run in 'boot.user'. Clojure only automatically refers clojure.core; we refer clojure.core and also boot.core.

So, dependencies notwithstanding, Clojure wouldn't be able to make sense of a build.boot file, and we think the .clj extension would be disingenuous.

We differ in these ways from .clj because one of our goals was to make it easy to create Clojure applications as single scripts.

The downside is users must configure their editors to get syntax highlighting, but we think this is the lesser evil.