Even a ghost might be scared of a vampire, maybe. She went off a cliff to avoid him before, so I sense a certain reluctance to talk. And what would they say to each other?
Real reason, of course-- they couldn't have him trying to convince reality and himself that Maggie was Josette if Josette's still hanging around. He'd address Maggie as "Josette", then the ghost pops in, abd Barnabas has to say, "Not you!" They wanted to do the Sara storyline, too. Josette's ghost was just a monkeywrench thrown into all that. Maybe I'm stating the obvious-- sorry.