That's because human generally don't like to
be the pivoting point for decision? Especially difficult decision?
Because the guilt will likely to reach and haunt her forever?
You see if your wife HAS to be the choice between herself and the
child, the decision is MADE between her life and the child's life.
If she decides to preserve her own life, it means her choice is to
SACRIFICE her child to keep hers. She won't be able to take the
consequence of such a decision and would rather chose to sacrifice
herself. This defies logics. This is emotional decision-making (or
Emotions > Logics ).
As from your point of view, you are weighing between a wife or a
child. And obviously, you would save your wife because you made
your marriage vow to your wife, not your child. She will be the
only person who will walk through your life with you, not your
children. This is logical. This is rational decision making. (or
Logics > Emotions)
I give you a simple illustration:
Say if your wife is terminally ill and wouldn't have much time
left. One day, came a drunken yunhaier with a gun, pointing
menacingly at the defenceless two of you.
Then I decide to play a ridiculous game.
'Only one of you is allowed to leave here alive.'
If I leave it to the both of you, chances are, your wife will come
forth (despite your resistance) and allow herself to be killed
because after all, she don't have much time left. (Rational
decision making)
BUT
If I personally ask you to make the decision, would you allow your
wife to be shot, even if she doesn't have much time left?
(Emotional decision making)
P.S: Nobody likes to make difficult decision. 
Cheers
