Because the NRL's uniform rules say that every team may have up to 5 NRL on-field jerseys each year, mandatory home and away jerseys and up to 3 other 'event' jerseys at the clubs discretion.
Apart from when a club chooses to use an event jersey, they must wear either their NRL approved home jersey if at home or NRL away jersey if away.
The home and away jerseys may only be changed once every three years, though teams can apply to change them early, and the NRL are pretty lenient about using other jerseys during the season even if the club has exceeded their 5 jersey limit if there is a good reason to do so (such as wearing a black/classic jersey when a former player/important person to the club dies).
But they are very strict about using the mandatory home and away jersey under normal circumstances.
We were wearing our official away jersey following the NRL's rule, so it's really their fault that we were wearing that jersey that created an unnecessary clash.
BTW, the V on our away jersey in green, yellow and blue, not just yellow and the teams were easily distinguishable on Saturday night.