Not many people want/need to study interstate so its not necessary. Plus factor in the administration and it'd be a nightmare. We'd have to wait even longer for results.
Yeah but I think if there was a national curriculum people would be more encouraged to go interstate. Right now it's a quite a hassle because of the different systems.
Not as big a hassle as you might think. A national curriculum would just save a couple of lessons at the beginning of a first year course, where the lecturer isn't sure of what background a student might have, so has to do an overview of all the basics - e.g. in maths, NSW don't cover matrices in the slightest, so they always have to catch up compared to, say, SA students, who have a pretty thorough grounding in them.
In any case, there'd always be international kids/IB kids who'd also have a different background too. So it's pretty unavoidable.