I believe a big part of the French downfall was an agreement between the codes not to take each others players in the early 70's, if you look through the French teams previous to this they contained many ex union players including internationals which helped the league team. I asked someone about this agreement and this is what he told me:
The agreement was called ‘The Protocol’ between the two (rival) French Federations - of Rugby and Jeu a X111.
It was introduced in 1972, agreed to to by the respective Presidents, Albert Ferrasse and Rene Mauries.
The basic agreement was that clubs from one Federation could not sign players from the rival Federation after they had reached adulthood, (I think this was the age of 20, ie. the age limit for ‘Juniors’) but if they were internationals at Junior level they were also subject to the ‘Protocol’ no signing agreement.
It was a deeply unpopular move among Rugby League people as they saw it as a Rugby Union ploy to stop League clubs recruiting Union players, which had been going on since Rugby League was first introduced to France – but, at the same time, Union clubs were very active in snapping up talented League Juniors as they came under the age barrier. It was seen as Ferrasse strangling a big part of League’s player-supply and also one of its biggest sources of prestige and publicity – and it was done with the backing and approval of the Ministry of Sport.
Look back through the history of French R.L. and so many players came into the game from R.Union. ‘The Protocol’ stopped all that.
‘The Protocol’ was finally blown out of the water in 1980 when the League’s biggest star, Jean-Marc Bourret, was poached by Rugby Union. They said it was in response to League signing a Union junior international (Serge Costals) so all ‘protocol’ was cancelled. And, once again, the government’s Ministry of Sport allowed the Rugby Union to have its way.