As sibling said, reddit was CL before it was Python. But more importantly, reddit predates Arc by more than two years. :)
And the main reason reddit was rewritten was library support. Doing simple things in CL was hard because you had to write it from scratch, but doing those things in Python was a simple library install.
It's easy to roll anything in CL if you know what you're doing, but making it talk wsgi or generate pngs isn't trivial, no matter what language you're writing it in.
Lisp shines for codegen and creating DSLs and new control structures, but if you're just writing out tedious imperative code, it isn't really better than Python. Its lack of popularity means it can't offer as many libraries waiting to be reused.
