Don’t think there is a good reason to keep lein but one of the reasons we keep it is that it all “just works” and we would not like to change things just for the sake of it. “Don’t change anything unless its broken” :)

One of the babashka maintainers here. bb.edn is for projects that run on babashka not for standard JVM projects. Since babashka itself is a standard JVM project we use tools.deps/lein as usual.