Given that as we learn more about various culture we once considered primitive and lacking agriculture, the more we find that they either practised agriculture in a way we simply misunderstood (Amazonian tribes for example) or consisted of populations that once practised agriculture but abandoned it (the Zomia people, are historians still convinced that Australian Aboriginals have never practised agriculture? Are there any academics looking into the possibility?
When I was at university one of the classes I took was in part on the Aboriginal People. This was at the Queensland University of Technology (my lecturer's name sadly escapes me). There were, and I guess still are, academics who were/are looking into this question. It was controversial at the time (this is about 8 or 9 years ago) and my lecturer mentioned that others at the university were dismissive of it, but he and others claimed that there was growing evidence that various Aboriginal Peoples of Australia had undertook some low levels of agriculture. This included growing gardens.
As to why it took so long one of his claims was that earlier in the European Settlement of Australia it was seen as better if the Aboriginal People didn't have agriculture due to the whole claim that the Aboriginal People had been too primitive to have a claim on the land.
Edit
Found a recent news article on the subject.