More for Melbourne people as this question comes up every year.
As said above, for a number of reasons a number of clubs give this tournament no emphasis, as in it's not even discussed.
I’ll avoid the inter club politics and religious components as well covered.
If you look at u18s as one example, load has a huge impact.
Factors such as:
Doubling up at either YL or PL/reserves plus u18
State champs in a few weeks
Classics a few weeks after SC
While the high end talent is there at u18 level, the clubs just don’t run as deep as across the border.
It would be great to have an inclusive well supported SA tournament, but for many, it’s a timing and load issue.