As far as I know, we only have speculation. But like volume 4 of Birds of Prey, and volume 2 of Azrael, one guesses there's a sales problem - while Tim's Robin series reprints are selling reasonably well (I used to work at a bookstore, and checked their sales records for these trades, and Tim sold about twice as many copies as Cass or the Birds, sadly), the other reprints aren't, so they're not planning on more unless there's a groundswell of sales or something. A real shame, since I think having full runs reprinted on glossy paper rather than the fading newsprint of the original hard copies would be really nice.