Pokemon Bank (as well as other new service Pokemon Transporter) were scheduled for release on December 27th 2013 but that was delayed for previously unknown reasons.
Now the two extra features for Pokemon X and Y have been released in Asia almost a month late, and a launch date for the rest of the world will be announced soon following some initial user testing.
This is the full statement released by Nintendo regarding the matter:
"Hi Pokémon fans, sorry it’s taken until now before we could give you an
update on Pokémon Bank. Pokémon Bank & PokeTransporter services have
now resumed operation in various Asian territories incl. Japan. We’ll
be closely monitoring players’ experiences as we prepare for the
European release. Once again we want to apologise for the delay, and
thank you for your patience and your interest."