It would be a massive undertaking for Mongoose to change over to producing plastics. Asside from all the additional equipment that would have to be brought in the moulds for the ships would probably have to be redesigned; which wouldn't exactly be cheap. Add to that the fact that they would have to continue producing the metal miniatures whilst this is change over is occuring. They would have to sink a large chunk of change into this effort, not an undertaking to be taken lightly.
Although then again they would be nice, especially in terms of some of the larger miniatures; considering how much lighter they would be for basing purposes.