Anecdotal but whenever I see a lot of diversity in a stack it usually means there are a lot of different preferences on the development team, or that the stack was developed over a longer period of time with many members (and their preferences) coming and going. While yea, any SE should be able to pick up and run with any language, it is quite possible their team no longer has the same experience in Clojure or alternatively has way more experience in their current front runner (Python, Ruby, whatever). If that's the case I think it's quite reasonable (at least from a business stand point) to use alternate tools.