Noel,
IPMS/USA is what I consider a supply-and-demand organization where the contest is concerned. There are a lot of A/C categories because they are the most popular plain and simple. As an ex-head ship judge for the society, Nationals chairman, and ex-NCC member, I can tell you our categories are based on what shows up on a consistent basis. The head judges for each category have yearly records for numbers entered as well as the type of models. When I was a head judge, if there was a consistent and potential growing number of say, Martian aircraft carriers over a three year period, I would put in a request to the Chief Judge that a category or split be added to next year's contest to accommodate the increase in those models. If the request was granted by vote of the NCC, the category was added on a three year trial basis. This was done to insure that it wasn't a one time occurrence, and could be removed if numbers went down for three consecutive years. Under-attended categories also face removal by the same system.
I realize this sounds like it would take some time to expand category numbers such as automotive, but that's the tried and true way IPMS/USA regulates its categories. Furthermore, the NCC must consider the cost to the host chapter when adding categories. Ideally, every category should have a sponsor which never happens; so the host chapter must foot the bill for un-sponsored categories from their profit margin.
In short, "build it and they will come".