The accepted reason is that in past times, ladies who could afford clothes with buttons on could afford a maid to help them dress, and it is easier to button another person's garments if they are button 'right to left'. A gentleman on the other hand would be handed his shirt by his valet, and button it himself. It is suggested that in the days when swords were worn, a 'left to right' buttoning reduced the chances of a sword being caught in clothing when it was drawn.
Most European countries appear to have the same convention. Differences in buttoning made it possble for women to wear masculine style clothing without being accused of sin or heresy....as there is a biblical injunction against cross drressing. (this was one of the trumped up charges they burned Joan of Arc for.)