Having had two children go through state programs (one made state teams, tho other didn't, always just missed out) I can say that my experience is.
1. The selection decision does not rest solely with the head coach - there are assistants & a selector.
2. Form in the East Coast Challenge plays a part.
3. Team mix is important - you can't have a team full of guards or a team full of bigs.
As unfortunate as it is with Basketball you are generally playing for one of two spots - starter or bench.
Finally, whilst making junior state teams is a fantastic achievement, the true test is where all these juniors are once they have finished juniors. Some are good in juniors because they matured early which generally means they have a size & speed advantage.
There are many cases of players playing in WNBA, WNBL, SEABL, State etc who never made state junior teams.
So if you child is good and loves the sport, continue to encourage them to be the best they can.
One casual observation - Bulleen have made the GF presumably because they are a good team. Good teams generally have good players.
Unfortunately Victoria is a strong state for Basketball, especially the girls so if you think you child should play Nationals, then you could move to country Vic or in interstate?
As an general example of Victoria's strength. Last year at the U20 Nationals Victoria entered two men's teams and they both made the final.