Because the Earth rotates west to east, no matter where on Earth you are, the Sun rises in the east and sets in the west. In the northern hemisphere it passes through the south as it travels across the sky*, and in the southern hemisphere it passes through the north.
The first clocks that displayed the time (rather than measuring intervals of time) were simply sticks inserted vertically into the ground (gnomon). As the Sun moved across the sky the shadow cast by the stick would move across the ground; at midday the Sun would be at the south and the shadow would point north, to the “twelve o’clock” position.
An interesting consequence of this relates to the convention of “clockwise”:
If the development of the first clocks taken place in the southern hemisphere rather than in the northern hemisphere, clockwise and anticlockwise would be in opposite directions.
* Hence why, in the northern hemisphere, a south-facing garden is an attractive selling point for a house.