Why are north facing slopes in the southern hemisphere warmer than south facing slopes?

For the same reason that south facing slopes in the northern hemisphere are warmer than north facing slopes. From the point of view of the Earth, the sun never strays farther from the equator than the 2 tropics. So in the south the angle of the sun on a south facing slope is always lower than for a north facing slope (in some places it might always be in shadow). For this reason (more direct sunlight) people put solar energy panels on north facing slopes in the south, and the opposite in the north.