Fridge Logic - The primary landmass of the world is in the southern hemisphere. Considering that north is defined as the celestial "top", this implies that either the first people to develop astronomy lived somewhere in the north that no longer has contact with the south, or when they developed it in the south, they considered themselves the celestial "bottom".
But just think about before the First Cataclysm. Istar, anyone? And besides, Solamnia takes up most of the north - which was the ruling civilization at the time. So technically, for most of Krynn's history, the celestial "top" was the North, and after the fall of Istar and the Solamnic Knighthood, people were just too lazy to flip the map over.
There is in fact another continent about the size of Ansalon: Taladas, in the northern hemisphere and just about on the opposite side of the world. There's hardly any contact between the two, and it's hardly been mentioned at all since the Expansion Pack World sourcebook that established its existence.