If it's QF72, I can somewhat see why Qantas won't offer PJs - it's a "dinner time" service essentially and the flight time is too short to get proper rest.
Even my last QF flight - QF36 SIN to MEL was only around 7hrs and even though that was an "overnight" flight (leaving SIN at around 8pm, arriving MEL around 6am), I didn't really have enough time to get proper rest after having dinner and before breakfast.
If you skip both meals, then maybe.
If on a 737 though I can completely understand why PJs aren't offered - at least on a widebody one could theoretically "change" into their PJs within their suite - on my QF36 flight there was one person who did just that - they pretty much put on the PJs over their existing outfit and then removed their "outside" clothes from underneath their PJs whilst at their suite - for the bottoms though they did go into the lavatory (thankfully). On a 737, half the aisle would be lined with people to get changed.