It is because ,
For Men, majority of population are right handed and find it easier to fasten the buttons if they are on the right hand side.
But for Women, Most of them were dressed by servants, as initially the dresses with button were only for the rich people. Thus for the servants (assuming the most of them are to be right handed) it will be easier for them to fasten the buttons by facing the Lady if the buttons were on the right side of the servant that is of the left side of the Woman.
