Mystara is more than just a kitchen sink setting. Most other D&D settings were "if it fits in the rules, there is a place for it in the setting", with Mystara it was more like "if someone can think of it fitting in the most bizarre fantasy, there is a place for it".
So you end up with
a flying aircraft carrier inhabited by gnomes, gremlins, and nagpa.
an empire run by a council of 1,000 (!) 36 level magic-users.
a nuclear power generator as the source of arcane magic on the planet.
high-tech "artifacts" all over the place ready to explode and cause the next cataclysm.
a city on the ocean floor with a fleet of magical submarines.
a hollow planet serving as the immortals museum of extinct cultures plus floating continents.
frequent enough explicit interdimensional or interplanetary migrations of human nations.
mixed in with bog standard low-magic nations and primitive cultures. Nearly all human cultures are explicitly immitations of real world earth civilizations.