I guess they didn't consider the market for Lisp books very profitable:
»We're NOT looking for: (...) Books on topics that have dismal sales despite quality books being available. (If you're addressing a topic where good books have sold dismally in the past (for instance, LISP, LaTeX, or Web-based training), you have a much higher threshold to clear with your proposal. Convince us why there is a revival of interest in your topic, or why your approach to a deadly topic will provoke interest nonetheless.)« (from http://oreilly.com/oreilly/author/writeforus_1101.html)
However, in 2009 an O'Reilly editor wrote: »I think there's reason to believe that's changing, particularly with the right book; LISP awareness has been growing for the past couple of years. Our Haskell book has outsold expectations; Erlang has obviously made a comeback; so there has been a relatively recent shift in the language map.« (from http://lisp-book.org/talks/eclm-2011-10-23/plan.txt)
Because it takes some effort to write a good Lisp book. There are several very good books: SICP, PAIP, AMOP, On Lisp, PCL, LoL, ...
Also typical Lisp hacker won't buy a slightly edited and printed version of the HyperSpec or a printed version of the SBCL manual.
Add to that that Lisp made a deep dive after the AI winter and that they were sure to not sell enough copies. At that time it was also a good idea not to be connected to Lisp.
Plus, I guess they did not like Lisp. 'Clojure' has the 'advantage' not to use 'Lisp' in its name.
FWIW, we didn't go to O'Reilly; they were shopping around for someone to write a Clojure book. So, whatever guidelines they have out there probably shouldn't be taken as actual "policy".
