Realistically it's a lack of cohesion, majority of Teir 1 nations players play in the same league such as England, New Zealand, Australia, SA etc. Fiji are strewn all over, less so now with the Drua but still more than other teams. It's also getting access to their players, Fijian players are often not released by their clubs, French in particular, due to injury risks. Not to mention they have some great players but also lose loads to the All Blacks and Australia.