The best explanation for why the Southern Hemisphere experiences seasons opposite to those in the Northern Hemisphere is:
The tilt of Earth's axis is reversed in the Southern Hemisphere.
However, this statement could be misleading as the tilt itself is not "reversed," but rather the orientation of the tilt in relation to the Sun results in opposite seasons. During the Northern Hemisphere's summer, the North Pole is tilted towards the Sun, resulting in warmer temperatures, while the Southern Hemisphere is tilted away from the Sun, resulting in cooler temperatures, and vice versa.