I reckon there's a lot of reasons, historically some are less than ok but that's our history in a lot of ways.
Eaier years, post the Indigenous tour of England and federation, cricket was a well-off man's game and a white man's game. I think that held on for longer than it should have tbh, or at least the perception, so that generationally a lot of kids the Era of Lillee and Border and all those still hadn't got into the game in a significant way. There were also other social pressure from that sort of time until not very long ago, especially to do with income and so on. So all that would have played a part.
I also think in recent times the cost has been prohibitive for a lot of families regardless of their cultural background. Cricket in general has shown a downturn at junior levels in most places tbh. Even if parents are happy to pay hundreds for fees, hundreds more for gear, and then the time spent as well, once these kids get to grade cricket they're paying thousands and many just drop the game or play socially. That goes for all kids, but obviously there are still social and cultural pressures on indigenous kids that other kids may not have.
That being said there are more indigenous players around than people might think. Alex Carey is indigenous and he's the ODI vice captain. SA and Strikers bat Jake Weatherald is indigenous. So is Dan Christian. Ash Gardner is a superstar in the women's game and she's indigenous. That's just off the top of my head obviously.