It is not just cultural, in the sense that the Nun order died out so it is historical, as well. In the days of the Buddha there were Bhikkhunis (nuns), Bhikkhus (monks), and lay people - lay people in white clothes, lay women and lay men. Five hundred years ago, I think, the Bhikkhuni order died out, there weren't enough Bhikkhunis which is why Bhikkhus, or monks, are prevalent today. The nuns wearing white in Thailand today don't take on as many rules as the monks.