I think it is because stallions can produce more, can be more of a hassle to handle and house, and because it is easier to end their breeding career that a mare's.
I don't think grade (talking crazy crosses, not purebreds without papers), mare or stallion, should be bred.
I have nothing against grade horses, some of the best horses I have ridden or known have been grades, to me its just a issue of numbers and over production of horses for the amount of demand.
RH Queen O Anywhere "Sydney"
2009 Sugarbush draft