From the announcement:
Nintendo and The Pokémon Company International have postponed the launch of Pokémon Bank and Poké Transporter - two software applications that were originally scheduled to launch for Nintendo 3DS on Dec. 27 - due to a large volume of traffic to the Nintendo Network service. Due to the high traffic, players are having trouble setting up Nintendo Network IDs and downloading content in the Nintendo eShop on both Wii U and Nintendo 3DS. We truly regret the inconvenience, and wish to reassure everyone that providing a solution is our top priority. We apologize for the delay and thank you for your continued patience.