Content licensing. Different companies (or legally distinct subsidiaries) control licensing & distribution for different countries. Inherent limits of physical books (i.e.: space & mass & energy needed to haul content across significant distances) mitigated the issue, as it's usually cheaper to get a book locally than ship it from afar. Now that content can, for most practical purposes, be shipped free with instant delivery the legal ownership rights are being enforced and controlled (and more middlemen squeezing into). Amazon will get around to sorting out licensing issues elsewhere, but 'til then they've got a big local market to satiate.