Lots of reasons
1. Australia has not had the need for carriers.
The last time Australia was seriously under threat of being invaded was WW2. That's more than 70 years ago. Since then the RAAF has been part of multinational operations in far away places - Korea, Vietnam, Middle East, ISIS etc. Australia has been in a relatively quiet and secure corner of the world. Australia decided to go with the JSF against this reassuring backdrop.
So until recently, we believed the RAAF would most likely be part of multinational coalitions contributing to multinational operations in distant places.
The need to defend Australia has only come to the forefront over the last 10-20 years with the rise of China, exacerbated by more recent concerns about possible future withdrawal of US from the Western Pacific.
2. Carriers are expensive.
Very expensive. Hence those nations with carriers belong to an exclusive club.
Most countries don't have any carriers. 4-5 countries (France, India, Russia, Brazil) have just 1 carrier. Italy and China have 2 small carriers. UK is working towards having 2 carriers. USA has 11 large carriers.
Its not just the cost of the carrier and the aircraft. It is also the supporting escort ships required to protect the carrier - missile cruisers, anti-submarine frigates or destroyers, submarines. Without these escorts, the carrier (with your planes) is vulnerable and won't last long. Plus you need supply ships.
Then you probably need 2 carriers to be certain of having one available when you need it. Carriers need periodic overhauls and upgrades, and these take 1-2 years to complete. Of the 11 US carriers, only 7-8 are able to be deployed at any one time - the rest are in overhaul. Countries with just one carrier have long periods where it is unable to be deployed.
3. Carrier operations are very complex.
The technology is complex, and the logistics are complicated. It is a very steep learning curve.
It would likely take a country like Australia 20 years to acquire the capability and knowledge to undertake carrier operations - and that is assuming money is not a limiting step (when of course it is).
4. We may be on the verge of a new era in military aviation
Will piloted aircraft still be relevant in 20 years time? Will carriers be considered too vulnerable to a new generation of high speed long range missiles and other weapons? Will carriers become a case of putting all your eggs in one very vulnerable basket?
Will we instead be using long range drones with new generation sensors and weapons? A drone is far cheaper and more expendable than a piloted plane, and range / endurance will be much greater if you don't need a pilot with all the human systems.
Investing in carriers now could be an expensive trip down a dead end.
It may be cheaper and more reliable to operate drones from a number of small bases in northern Australia - we can acquire this capability faster, and this dispersed capability will be less vulnerable to enemy action.