The current 20kts runway nomination criteria came about because QF 767 drivers pushed for it, plain and simple. It could be 15kts, it could be 30kts. ATC don't care but play by the rules. The problem is that there can be 15kts at the northern end of the field and 30kts across the bay, therefore 25 only.
If the pilot union(s) want a higher crosswind criteria then have a word to CASA.