I don't think it has anything to do with anyone's particular interest in the characters per se.
It's the novelty of seeing these characters - which are incredibly well known - again at all, and especially interacting with other characters that are quite well known.
It's the same reason JLA/Avengers did so well, why Death or crippling or resurrection once increased sales. Superhero comics have exhausted almost every avenue -- literally exhausted them. Bringing the Watchmen characters into the superhero universe is perhaps the only big, recognizable PR stunt thing left to DC.
The closest cognate - say, making Sandman a regular member of the JLA - wouldn't even come close, and would have significantly more obvious material drawbacks.