Without getting bogged down in the details, many of which I don't know anyway, US publishers tend to be more conservative than in many other parts of the world, and less willing to take on titles that do not fit into the major FPS/RTS/RPG genres. Indeed, with the high fixed costs associated with a run of DVD production and packaging, it's increasingly hard for publishers in many countries to justify releasing games that way unless they can be pretty sure of selling millions of copies.
If you live in a country that doesn't have a publishing or distribution deal, then there is nothing stopping you from ordering the game from an importer, or from a supplier willing to ship internationally. The Egosoft shop does this, as do various retailers. You don't even have to order the English version. If you can only find, say, the German version available for international delivery, then the only difference is in the packaging; the contents of the DVDs are exactly the same.
Oh, and as always: please take general discussion of the pros and cons of Steam to the extensive Steam thread in the X Rebirth forum.
