Pourquoi ma demande de réservation a-t-elle été rejetée ?

There could be several reasons why the landlord/-lady rejected your booking request. Perhaps the confirmation of your booking would cause a gap in the booking calendar of the apartment, the currently ongoing rental agreement has been extended or another booking request seems more suitable to the landlord/-lady. It is also possible that the apartment is being used for another purpose and the availability has been updated accordingly. 

More information: Can I influence whether my booking request is accepted?