Politics and cost. The present Australian government has long held that we are best seen as the "White Ghurkas" of the Pacific - imperial sepoys to which ever empire we have decided to hitch our wagon to. Therefore, it is better to have the same equipment as our "great and powerful friends" than to necessarily seek the best equipment for the particular role. Often, we will hold "competitions" to determine which is the best but invariably the ultimate decision is a political one. In the 1960s, we ended up with the M60 GPMG, after the FN-MAG58 beat it hands down in competition. 30 years later, the FN-MAG58, replaced the M60 GPMG in Australian service.
Today, we find ourselves with the M1a1 AIM. While a fine tank, in itself, it is not the appropriate one for our needs - nor is the Leopard 2. Both are excellent MBTs however both are heavyweights and our national and regional infrastructure cannot support the movement of these vehicles. Our roads have insufficient, strong enough bridges, which can carry the weight of both the MBT and its transporter. Many of our bridges are not high enough, to allow the MBT on the back of a transporter to pass under. Our main north-south railway, which was built, ostensibly because of defence needs (after nearly 80 years of shilly-shallying and a world war when it would have been particularly useful), cannot carry the MBT because of bridge and axle loading concerns. Most other railways have tunnel and bridge loading problems, transporting the vehicle. Only three ports have cranes heavy enough to allow the loading/unloading of these vehicles from ships and only two ports have wharves strong enough to allow ro-ro loading/unloading. Conditions within the region - Oceania and the SW Pacific, are even worse, with few roads or wharves able to take these vehicles. We will have insufficient air transport capabilities to move more than a troop of vehicles at one time and few runways will be strong enough to take the aircraft.
Finally, there are few MBTs in our region. Those that are here, are ostensibly owned by our major allies there. Therefore there are few targets that need a 120mm round and few threats that cannot be faced by a lighter vehicle.
The sole reason we are buying them is because there are no more medium tanks being produced, outside of Eastern European designs, which come with their own raft of problems associated with them. Further, tactically, because the M1a1 AIM utilise a gas turbine engine, their POL requirements will skew any operation in which they are used, particularly at a time when the Army has run down its logistics.
All-in-all, a poor choice IMHO. They will be parade-ground queens for the most part, I believe. They are intended to provide at best, training of Australia's armoured regiment (although the numbers will be insuficient for that, according to the information I have) so it can then be moved overseas en masse to be America's Ghurkas.
