The first reason is that few in France know who a Sikh is.
Sikhism, the youngest monotheistic religion, is unknown to many. It is a religion from the North of India (and specifically from the Punjab) whose inhabitants are little seen in France until recently, the Sikhs only being in Paris significantly since the 80s (for a good 200 years among the English by colonization).
The second is that the turban (Paag) is more than just a hat.
It is wanted by the last human Guru, Gobindh Singh to differentiate Sikhs from Hindus and Muslims with the 5 K, symbols of Sikh (the translation is long hair, comb, steel rings (Kara), special pants (Kaccha) and a sword).
This is done in the context of the establishment of the Khalsa (the pure community). India then saw the decline of the Mughal empire (Muslim empire) which was crumbling in the corruption and oppression of other religions.
The Sikhs are then in full rebellion against the empire which persecutes the Hindus and have executed the father of Gobindh, Guru Teg Bahadur who had gone to Delhi to ask for the stop of the persecutions (of the Hindu priests of Kashmir in particular).
The turban was then also a marker of social class, Guru Gobindh Singh then proclaims that the Sikhs will wear it to symbolize their equality.
The third and last reason and for a practical aspect.
Sikhs are required not to cut their hair (see earlier). God having created the body of men as well. The turban therefore makes it possible to contain all of this. It’s also a comfortable hat that protects from the sun.