Just off the top of my head,
It would take up space that could be used for three or four paying passengers. This would result in increased ticket prices for everybody and the non-smoking majority have no incentive to subsidise a smoking cubicle.
It would require a ventillationventilation system that would prevent smoky air from getting into the rest of the cabin, which would add weight and complexity.
It's a fire risk and a small fire from some idiot disposing of paper in the ashtray could spread; it would be hard to detect such a fire early because you couldn't just put a smoke detector in the smoking cubicle. (I guess an infra-red detector would work.)
Almost nobody tries to smoke in the lavatory so reducing the occurrence of an event that hardly ever happens isn't worth much.
Rates of smoking are decreasing in most Western countries, so demand for such a cubicle is already falling.
Anti-smoking legislation is often aimed at worker protection: who's going to clean the smoking cubicle and what equipment will they need?
I doubt there's significant pressure, even from smokers, for such a service to be provided.