My understanding is, as Omar said, they become the practical choice for women working at the home front during the first world war, something which simply doesn't done until then (ala the expression 'Who wears the pants in the relationship.') As such they became a symbol of female suffrage during that time, and were revived during the late 60s and early 70s by the 2nd wave feminists; their popularity has simply stuck. Also the rise in cheap clothing means that women can get flattering jeans and the like, while still retaining the durability and ease of wear (they've also become part of the unisex 'casual' uniform).