When RL held its first World Cup, GB, France, NZ and Australia were the attendees. Perhaps other countries could have been included back then but either due to cost or lack of a competitive standard that didn't happen. So for many years, international RL was about four countries.
In 1995 the WC went to ten countries and since then some effort went into trying to expand the international game. The problem was the top three countries were lifting their standard and while France fell behind, other aspiring countries were not breaking into the elite circle. To be competitive, players have to participate in one of the three professional leagues, Australia, England and France. So how were the two hemispheres evolving internationally?
Northern: GB became England and with English players of some ancestry in other UK nations choosing to play for them, there was a reasonable pool of competitive countries. England was above the others but France, Ireland, Scotland and Wales had a decent standard.
Efforts were made to assist countries. Italy and Lebanon would draw on Australians who qualified when a World Cup came around. North America has tried too. However, apart from England and France, the other nations have not developed strong domestic competitions and in many cases have slipped backwards in recent times. France has also struggled domestically in recent times leaving England the sole strong Northern nation.
Southern: The game Down Under was dominated by Australia with NZ occasionally causing an upset. PNG is mad about the game but struggled to translate that into becoming top tier nation with a domestic competition not of a high enough standard.
Pacific Islanders moving to NZ and more recently Australia have taken to the game. However, the best of these players were picked by the top two countries and the Pacific nations couldn't compete.
Something changed a few years back when players of Pacific heritage started to play for the island nation their parents emigrated from. This slightly weakened Australia and NZ but massively improved the Pacific countries. Now there are several Pacific nations that are competitive.
Summary. There are currently about eleven countries that are of a reasonable international standard, with heritage players supporting most of these. The imbalance is the Northern Hemisphere provides just four of them and two only for World Cups. England looks south for competition as the gap widens with France.
So on the one hand the international game is in better shape than it has ever been but it's basically a Pacific region success story. It seems the game cannot expand on that situation so internationally this is as good as it gets.