Like many polo traditions, this one can be traced back to India, where the game was played by nobility and later introduced to British army officers stationed there in the nineteenth century. Competing in the intense heat, players preferred clothing that was light in weight and color.
One team differentiated from the other by using the numbers on the front of the shirts and the other, on the back of the shirts.
Nowadays, although shirts are designed to identify each team as a brand, white pants are tradition and part of the polo culture.