Our jersey's throughout the pre-season in the 1990's were exact copies of strips that Wigan used during the English season. At the time their jersey's often involved curves or unique patterns, requiring the synthetic material.
Also, as explained by Smithtown, teams would wear synth strips during the pre-season due to the old "they are cooler in the heat" thought pattern, with cotton being the more durable tried and tested jersey for a long season of footy. They were also substancially cheaper in a time where all three grades wore the same jumper with sequential numbering through the grades (1st Grade 1-13, Reserves 14-26, U21's 27-39, Interchange 40+).
As for St George sticking with cotton longest, I am pretty sure that is true, though Penrith stuck with cotton for a long time into the 00's as well. Our jerseys changed over around the time when we changed our sponsor to St George Bank. Supporter wear didn't become synthetic until Adidas started making our jerseys