Good question. It may have something to do with local laws (DRM), or even having to translate everything into that country's language (and some countries have more than 1 official language--Belgium and Switzerland, for example). Ever use Microsoft Word in France? It's surreal--you see the same icons, dropdown menus, etc, but of course commands are in French.
Mainly, it would be because B&N doesn't have a shop for each country.
Remember, the iPad and Nexus are sold mainly as general purpose tablets by companies with global subsidiaries. B&N is primarily a US-based /bookstore/, and the Nook is sold as an e-Reader (with built-in B&N shop).
Due to contracts, B&N may not be able to sell books in another country -- especially if the publisher hasn't released the print version of the book in those countries (not to mention changes like UK: Philosopher's Stone vs US: Sorcerer's Stone). B&N would have to 1) have a presence in those countries [eg; a "shop" specific to the country] and 2) obtain contracts with the publishers to match the releases of those countries. Then the Nook could be registered to the local subsidiary [determined by CC number and billing address] and access the shop. Just look at the hassles the UK release is encountering.