Simply for the sake of our (the staffs) work load Maarten. All those armies had to be built into tables, and when ladders shared common themes we grouped them together to save time and effort. The obvious answer is usually the correct one.
At some time in the future, the armies, as we currently know them, will be going away. A rank structure will be maintained, but instead of picking an historical army your profile will display your national colors (or the colors of a nation you select if you prefer), and because of that, we won't be tweaking the existing army system.
Paul