I imagine that at the moment we are really talking about the economics of running an event for a much smaller industry. The marine industry in Australia is worth about $7 billion annually, the pinball industry I have no idea, but I am guessing $100 million??? So there is an issue of affordability - the larger the industry the more easily they can afford a great expo.
Most industry expos are run by professional exhibition organisers who do it for a living and I presume they make decent money out of it as they usually charge both the exhibitors and the exhibition-goers. I have seen certain exhibitions cease over time due to eventual lack of interest from the industry.
As to last expo - there were a number of exhibitors although the size of the venue did not allow too many. I think it wouldn't have been too difficult to get them back as they seemed to do all right out of it.
