Thanks for your answer.
During those 8 years (minus 1 year of WHV), I completed 2 diplomas after that, I was the secondary applicant for 2 more student visas. From my last diploma to the new one I was applying for, there is a gap of almost 4 years.
Over the last 8 years, I went back to my country only twice for a short period of time. Apparently, it means that I have no significant ties with my home country... Even though my parents visited Australia every years for Christmas over the last 7 years (which I did not explain in my application).
When you enter Australia and New Zealand, they ask if you were ever refused a Visa in another country. Apparently in USA it can they can even refuse a simple tourist visa based on your immigration history(?)