Polo shirts originated in19th century. Originally woven from worsted wool, it was originally called Jersey shirt (also synonymous with sweatshirt) because it was woven on Jersey overseas in Britain. It is used in football, boating and other sports.
In the Victorian era, Polo sports suits included white flannel pants, a white tennis sweater (to cause sweating) and a white shirt with a neat neckline (which was later buttoned).
Later, jerseys gradually replaced the old Polo shirts. The new shirt has a low collar, lapel or lapel, long sleeves and five buttons in the center of the front collar. This new jersey has long been popular in the polo world represented by Richmond Park and the British Polo Association.
The above is taken from Baidu Encyclopedia, polo shirt entry.
Generally speaking, because Polo is called polo, people first wore this kind of clothes to play polo, so the clothes are called polo shirts.