Honestly, I think the biggest problem is just a lack of opportunities for players. There are only so many places at Catalans, and so the choice for young players is either to stay there and hope for an opportunity at some point or take the initiative to try and move to an English club, which again is really just a shot in the dark. English clubs are insular enough as it is and the thing is that French players are unproven compared to NRL imports or even English players from lower divisions who represent more of a known quantity and don't have the language barrier etc. For French players to make it with English clubs they would either have to be established performers at Catalans, or be willing to leave home and pay their own way to move to a different country with a different language at age 17/18, and then spend a couple of years playing in the junior grades for virtually no money on the small chance that they might get a full time contract at the end of it. The odds are just stacked against them at the moment.
Player drain to Union has been mentioned but it isn't really that much of a problem TBH, I can't really think of any top prospects that have left the sport since Estebanez about 10 years ago. Most of the junior players just end up fading away because they really have nowhere to go. More French players have been signed by English clubs recently so it's starting to become less of a rarity but it will still take time until they are considered a true viable option by English coaches. France would be much, much stronger for a second Super League club giving a full-time pathway for twice as many players, it isn't like the NRL where there are large numbers of NZ/Pacific expats already living in Australia.
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