The reason behind this is that the LFP who organize French football wants to have 3 professional division. In France we have 2 at the moment and the third division is semi professional. By reducing the number of teams in Ligue 1 and Ligue 2 they want to professionalize National which is the 3rd league.
It has since been expanded, and while most of Europe's national leagues can still only enter their champion, the strongest leagues now provide up to four teams.