It's not just Ligue 1, I think this always happens when you have a team from one of the smaller leagues that has a "golden generation" that over-performs as a unit. More often then not they are all quite young, and have played together for a while, has a manager that manages to get the best out of them as a team, etc. Then when they split that, they individually don't hold up to the task required to change league and team environments (to a higher lever, as well).