Ayane begins Razor's Edge by slaughtering a soldier and his mates wheh he tries to to rape her. Ayane is a Child by Rape and not only was she shunned for this her father was such a bastard her sister set out to murder him.
The recent Flip Flop of God regarding the chronological order of the games leads to this. He beginning of the first Xbox game features Ryu in traditional ninja apparel, but after Hayabusa village is attacked, he switches to his modern gear and sticks to it whenever he needs to go on a mission or save the world. Yet in the NES games and the OVA, which have been confirmed by Word of God to take place after the Xbox games, he's back to wearing more traditional armor again.
Not sure about the other 2 NES games, but the first does have an explanation. Ryu didn't expect to need his black outfit when he went to visit Walter Smith, because he had no reason to expect he'd be attacked. By the time Ryu's fighting off the forces of evil, he doesn't have time to head back to the village to change clothes.