Why do womens' side-zip skirts and trousers zip on the left side instead of the right?

The side zip actually used to be on the right. This is because the bulk of dressmakers were men up til the 19th century and they werent the ones wearing the dresses obviously, so women were also accustomed to wearing garments with this side zip. But basically, it was realized that most people are right handed so the act of crossing one's hand over to the left side was realized easier so in convenience for the right-handed majority, it was switched over in the recent 20th century.. We were actually discussing this last week in our collection development class!