While in theory north and south being portrayed as up and down are arbitrary distinctions - we could have easily drawn the world map the other way up - one big reason we picture them the way we do is that most of the land in the world is in the "top" or northern half. The Northern Hemisphere is everything from the equator to the North Pole and contains most of the world's population as well as the majority of its countries.
There are theories regarding plate tectonics as to how this became the case, but it might just be arbitrary since landmasses do move very slowly over time. As a result, the map we know and love has been much the same throughout human history, moving a couple of feet at most. That means the equator has relatively little land on it, as it passes through South America, Africa, and Indonesia, meaning that the overwhelming majority of its span lies on the ocean.
The Northern Hemisphere is thus home to most of the economy of the world, and most of its more dramatic topography, from high peaks to low valleys, from deserts to tundra to endless boreal forests. It also currently contains the Arctic ice cap, though trends indicate that probably won't be around for much longer. Let's see how much of the detail of this most happening hemisphere you know!
