Wearing men's attire had to be at least mostly out of practicality. For one, men's clothing would have been more available, and secondly pants are the only way one could endure very long rides on a horse. There are casual photos of her wearing a dress. I assume she mostly dressed like a man when riding. It was probably also out of safety, from a distance in that clothing she would have appeared to be a man and to an outlaw potentially less of an easy target to get bushwacked.