This has been covered ad nauseam across so many different rugby forums.... you should know the reason by now.
Other than not having the test match at all (not an option) - there was literally no other time this could have been played.
The scheduling of fixtures is mostly controlled by the broadcaster (9) who also have the NRL and the Olympics. The Olympics understandably take precedence, and NRL broadcast contract is rock solid - so it rules out having it Thursday/ Friday night or Sunday afternoon. Channel 9 would also care less about community sport than the commercial value of their product/ ratings, so putting it on at the same time as any of the above on a secondary channel isn't an option.
Saturday afternoon is the only time it can happen. QPR clubs have extended a wonderful olive branch in moving their fixtures, unsure if GPS schools can do the same but sounds like they can't either (understandably considering they have 5-10 fixtures per age group playing across multiple fields on a singular day).
There. Covered. Done.