I was told a story last century by a senior public servant, which suggested that the government of the day was under pressure by some donors to the party, not to explore legalisation.
I wouldn't be surprised if the ALP still has such pressures[we are talking state level by the way].
There are certainly groups in our society, legal and otherwise who benefit from the present situation, but basically I would still see the main thing maintaining the present system, as years of indoctrination that the "war" on drugs must be won, when it was lost ages ago.
