Air France replaced the Fokker 70 because of performance issues, mainly related to braking in wet conditions IIRC. These problems were the triggering event to join forces with Cityjet who had a more suitable aircraft available.
By the way, the Fokker 70 needs a few modifications for LCY ops, mainly software-wise. the onyl aircraft modified as such are the ex Air Littoral Fokker 70s which are flying with Regional, if memory serves.