At first to me it seemed a little odd, but I listened to an interview with Squadron Leader Paul Simmons, he explained that the aircraft contained toxic material and rather than recycle them, burial was the safest way of disposal. There has been some cases of RAAF ground personnel having major health problems having worked on the aircraft. I believe some were terminal.
Not all is lost as six aircraft are to become gate guardians, another six go to public museums in Australia and another aircraft is going to the Pacific Aviation Museum in Hawaii.
You will find a link to the interview with Paul Simmons here:
Guests - List of Conversations guests - ABC Conversations with Richard Fidler