They actually rewrote it in python because in 2003 open source implementations were of much lesser quality, also, hunchentoot didn't exist, and neither did the modern lisp frameworks like caveman/clack or restas. CCL and SBCL are way better than CMUCL and clisp, which is what I believe they used(CMUCL is barely used these days, and clisp has a bunch of rough edges and being byte compiled is slow, unlike ccl and sbcl which are pretty fast). These problems are no longer valid, and if reddit were to be written today, it would be less likely to be rewritten in python. Lisp has gone a very long way in the last 10 years.
Using common lisp and restas + Postmodern(for postgreSQL access) today is actually a better choice than using something like flask or sinatra. And since i dislike full stack frameworks, my personal opinion is that it's better than django or rails too, for a small team of good hackers.