I used to work at ITA. I seem to recall Carl admitting at one point that the choice of language had mostly to do with the fact that he and Jeremy were most familiar with Common Lisp, having used it while pursuing their doctoral degrees, and that he probably would have chosen Java in retrospect (this was in 2007). The actual code was not the clean functional code you might expect (and as the email above notes, much of it was also written in C++, half by line count IIRC). In fact, there was interest in parallelizing parts of QPX (the low airfare search engine in question) to speed up search, but it was not amenable to such changes without first having to untangle a bunch of single threaded side effecting code and without incurring penalties in performance or resource consumption.