Women's clothing typically buttons and zips on the left side because, historically, women have had assistants to help them dress. Since most people are right handed, left-positioned zippers made it easier for servants to help women get dressed.
Historically, men have always dressed themselves, and since most men are right-handed, it was a matter of practicality to make clothes with buttons and zippers on the right side. Since women (particularly wealthy women) wore more elaborate clothing than men, they often had servants help them get dressed. Having buttons and zippers on the left side of the garment made it easier for the servants to button or zip up the dress, since the buttons or zippers would be on the servants' right side.