The reason seems to just be that LISP books generally don't sell well. And it seems to still be in their submission guidelines.

http://oreilly.com/oreilly/author/writeforus_1101.html

"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),"