Why do sikh men wear turbans?

Sikh men wear turbans because it is an important part of the 5K's

Kirpan (A small sheath/knife for defence purposes only) Kesh (The hair) Kanga (A small comb kept on the user for the hair) Kacha (A pair of shorts which represents modesty) Kara (A steel bangle worn to show constant learning)

Also it is worn to preserve the Sikh identity, take care of hair, promote equality. Turbans serve the purpose of identifying Sikhs from people. Most Sikhs accompany a beard along with a turban which would abide by the rule to not cut hair. In Sikhism, hair is regarded as a holy object given by God and Sikhs believe in order to take care of it you must not cut your hair.