Yeah, the social and economical factors are the biggest hurdles for Aboriginal children. In Evonne's case, she was helped along by a benefactor who saw her talent and decided to invest some money in her. But Evonne also had desire and it was never mentioned to her that she couldn't achieve what she wanted. She also came from a country town where she had little contact with an "us against them" mentality and didn't have that sense of worthlessness that is instilled in a lot of city-dwelling Aboriginal kids. You'll notice that most of the Aboriginal boys who play AFL come from country towns also where they are mostly isolated from them white folks and didn't face discrimination on a daily basis.
Culturally, Aboriginal communities are laid back and the work ethic isn't the strongest (sorry, it had to be said but I think it's true). Evonne herself was always said to "go walkabout" mentally in the middle of matches. Even Cathy Freeman looked like she'd rather be elsewhere a lot of the time, even at the height of her track success. She comes across in interviews as a laid back sort of person and incredibly easy-going, whereas most athletes were more intense and determined. It's a generalization about the lack of Aboriginal success in tennis but it's a theory.
Didn't Evonne have a nationwide tennis programme where she introduces tennis to Aboriginal kids living in remote country areas? Has she unearthed any talent from this?