I think they are learning from the well known Hermes model. Less supply = more demand. This may also be just a short term policy until the brand gets large enough to just keep stock flowing. I think it is working. Every bag that comes into Barneys or Bergdorf is a precious commodity and sells within a few days.
I agree with the less supply = more demand thing. As I understand it, Phoebe Philo has been given a certain number of years by LVMH to establish the desirability and prestige of the brand (which she has certainly done an amazing job of so far), after which they will branch out into making their products more accessible - perhaps more affordable and more available. I don't know if that means they will introduce additional products that will be more affordable/available or if their entire range, including the pieces we're currently familiar with, will become more affordable/available.
I guess at the moment the mini luggage is the most in-demand item and is slightly easier to get hold of than other Céline bags (but still kind of difficult, especially if you're after a very specific colour/leather or don't live near a decent stockist), so items like the nano are proportionally more difficult to get hold of and seem more scarce than the minis.