In much of the Middle Ages, Aquitaine and to some extent the whole "Langue d'Oc" was linked as closely to Catalonia as to northern France, and they were flourishing probably more than northern France, in cultural terms. So while I've read very little about the historical evolutions of French - Dom could assist here - it seems to make sense to me that it remained close to Catalan in those centuries, and started to diverge more only when Paris and the north (the "Langue d'Oil" became dominant in cultural as well as political terms - in the south Occitan still remains, and it's still very close to Catalan.