Only because the rules are so weak, its easy for a player to represent more then one country. Origin isnt a scourge, the international rules are.
The ARL make the rules for Origin, you must be eligible for Australia to play SOO. Its one of our tools we use to develop players for our national team, why should we help other countries by allowing their players play in it?
The only fair ways for Fiji to keep Uate (as an example) is for the eligibility rules to be tightened (an RLIF problem) or for Fiji to have more meaningful matches (also, and RLIF problem)
The rules are not weak, its the administrators in there that are weak. Australia basically does make the rules fro eligibility and they tend to heavily advantage them. They have gone and done everything to help boost themselves up.
Looking at the RLIF who actually is in the positions of power there:
Chairman: Colin Love (ARL/NSWRL)
Vice-Chairman: Richard Lewis (RFL/ESL)
Secretary/Treasurer: David Gallop (NRL)
Member of Exec: Ray Heffenden (NZRL)
See any conflict of interests there... I do. Its why all the other nations keep getting screwed. These guys will bend and break rules to suit themselves. They don't provide any viable competitions for the lower tier nations where they have an opportunity to play the best players, so they naturally want to switch to the Aussies, Kiwis and Pom's.
So we end up in a catch 22 situation where the top 3 don't want to play the others because they look at them as easy beats, so the best players from the other nations want to swap to get to play the best and weaken their own nations, these nations don't get games against the top 3 because they are considered easy beats, so their best players swap over to them because they want play against the best etc.... round and round and round we go.
Origin is not a jewel as far as I am concerned, in its current form it is a cancer on Rugby League and only serves to do damage to the sport internationally. SoO and NRL is driving League further and further apart from the rest of the world and very soon the NRL will become like AFL, a sport unto its own that only QLD and NSW play... Australian Rugby Rules ARR is a good name for it I think.