Well the short story is, because it is cool. There are so many things you can do when sandboxing code is dead simple. That is what I hope clj-sandbox will do, allow clojure code to add sandboxing with less then 5 lines of code.
Of cause that is a idealistic view, to really customize the sandbox you might need 10 lines ;).
Long story? I need sandboxing for a project, and on #clojure hiredman, bsteuber and me got in a talk how nice it would be to have a simple sandbox library. So the next day I decided that I take it upon me to make it reality ;). Truth be told, hiredman did a very good job for clojurebot, I lot of code and ideas were pioneered by him :)