Why did Jack Thompson do this movie?

I would say he was fairly well established in Australia, and only just getting noticed by international directors and producers let alone audiences.

Australia was a pretty tiny pond in those days. Actors could be quite famous nationally but not earn a lot of money. I was told this by a family friend who worked for an accountant who had built up a clientele of show business people. She said there were household names who if they had a good year would visit and agonise about whether they could afford a very basic model new car.

They were the ones who were organized enough to have accountants to advise them and chase late-paying employers. Meanwhile they survived by never refusing a part unless already booked somewhere else.

Some were not well-managed and every so often on Australian current affairs TV there will be a story about some old star who is now on welfare.

JT is quite probably quite well-off now because he keeps working regularly in Hollywood and in all sorts of roles.