If you ring Shane Harris at Rare Spares (03)93054000 you might get the answer.
These were made overseas and it is possible the sales had slowed down. Getting the minimum quantity made at the factory might equate into the stock sitting in the Melbourne warehouse for a long time and tying up funds. These have been around for many years now so there would be a good sales history to make a decision on their future.
As with most manufacturers they want to make enough product to cover their machinery set up and make profit. This quantity they require assists in making the decision on placing orders and carrying stock.

Petrine.
You are correct in noting that the mufflers and tips are made overseas. The main reason is that no one was prepared to make them locally as a complete unit. Secondly the factory chosen produces thousands of exhaust components annually and had the ability to make a fitment jig. The sales volumes proved to be extremely low and required a lot of investment for a slow return. It is better to put the money back into products that are more popular and required by many enthusiasts. Selecting which items is always hard as you can't please everyone all the time.

One other issue is that when a new product is put on the market some people think they will be there forever. As you can see nothing is forever so if you can it is better to get those harder to get parts sooner rather than later. Of course there are some parts that will be ongoing but then there are many factors that determine supply. Suppliers closing down, tooling becoming broken or lost, raw material and labour costs increasing and even Mother Nature throwing in her hand.
Hope this answers your question and explains the situation clearer.