I imagine it's all down to how the original agreements were worded. Dark Horse can publish pre-DH Star Wars
comics, but Bantam seems to still have the rights to its old Star Wars
novels. (And its old Star Trek
s I think, though I don't know the last time I actually saw a copy of The New Voyages
in a bookstore.) On the other hand, when Del Rey got the Babylon 5
novel license, they republished a couple novels from the old publisher. In the Doctor Who
world, BBC Books (an imprint of Random House) can't reprint a Doctor Who
novel from Virgin without specifically getting the rights to it.