It's also just going to get worse with the new "cleanfeed".
In fairness, Europe and US have large populations accessing data over many links, so the cost is spread wide. Australia accesses much US data over few links and must bear the cost of that. Data is simply more expensive in Australia.
The telco's have probably had discussions on how to make local data not part of a cap, but it would be too hard to explain to normal people how to ensure that the data is locally hosted.
I think it's actually the capacity of the mobile network. To support more throughput they'll need to install more cell towers, which is a capital expense. The telcos probably find it more profitable to restrict usage and just charge the higher users as much as possible for as long as the network will sustain it.