Why Nobody buys 100mbs
Because it is not useful.
Not many people have a dozen ultra high definition TVs at home that then need to run concurrently.
25mbs is not very useful either. But it is priced only barely above the 12.5mbs, and 12.5mbs is not even available by Bigpond. If there was an option for 5mbs at half the price of 25mbs I reckon it would be taken up by half of all consumers.
That was always the hole in the concept. There are no applications, real or imagined, that need high bandwidth in homes. You do not need a 25mbs to file a tax return or access health records etc.
Meanwhile, people without any internet at all wait and wait in vain.
The NBN does have a plan to ensure that people use their services though. Cut the copper, and give people no choice.