The standard "chain of command" in the medical decision-making process is this:
Other blood relatives
Several reasons, as I see it.
It is not the "natural order" or things for parents to have to bury a child.
Marriage is a legal and sacred bond, made by conscoius choice and with consent (Las Vegas marriages aside
) You can't choose your parents - you can choose your spouse.
We have put a lot of weight on the sanctity of marriage in this country (U.S.), and it's sacred bonds. When you grow up and get married you are creating your new immediate family and your parents and siblings become your extended family.
On your death, your estate is immediately the property of your spouse... then your children... then any other blood relatives. Standard next-of-kin order of things. It makes sense for the medical decision process to be the same.
The most important one to me is the "natural order" of things. I think we all agree how sad it is for a parent to lose a child -- but it seems more "natural" for us to bury our parents. We look up to our moms and dads for guidance as we are growing up, so we would expect dad to make the decisions for mom, or vice versa. If one parent had already passed on, since we grew up raised by them, it makes sense that we could make educated guesses based on similar values in the decision making process. Then, if all else fails and there are no adult children, obviously mom and dad or other blood relatives are next in order to make the legal decision.