Mystara is quite popular outside the US because it was the first setting translated into other languages. If you wanted to play D&D in Japan you had Mystara or nothing. The allegory for real-world cultures as fantasy nations gave it an appeal that other generic fantasy settings didn't have. You would market it as the D&D setting that represented other cultures, so the Japanese, Italians, Polynesians, Indians, Arabs, Scandinavians, Slavs, Mongolians, Cherokee, Irish, Egyptians, Argentinians, Spanish, Babylonians and Texans all have something they can relate to.