Because as ground based bipedal animals, we move and live along a horizontal plane.
Separate note - while i get that north-up is just a convention and a mental construct, the south-up map really threw me off. As i was staring into the map, I felt the urge to turn my monitor around and had the same visceral need to set things right as i would upside down on a roller-coaster. This caught me completely by surprise because i was expecting to just make a mental note that this was a new convention i could get used to. Instead i "felt" uncomfortable - did this happen to anyone else?