Because a baby isn't a commodity to be bought / sold.
And because the article was all about the parents, not about the human being they are going to produce who may not self identify with the sex the parents have chosen.
My parents wanted a girl, my paternal grandparents wanted a girl (they only had boys), I was born a girl.
But I'm not the girl they wanted. I have been and continue to be a great disappointment because I have not lived my life the way my family thinks a girl / woman should.
I have never walked down an aisle to be given away by my father, according to my mother this is something he has dreamed about since I was born.
When I meet up with my parents if I'm wearing a skirt I'm told I look nice, but only if I'm wearing a skirt.
It took me a long time to realise that it was not my fault I was a disappointment it was their fault for having certain ideas about women.
I could understand wanting sex selection in historical times when a boy would inherit.
But these days, apart from work on a submarine, or actually give birth yourself there is virtually nothing that your sex stops you doing.
So why have a preference? Because of gender. Because some parents have very fixed ideas about male / female roles.
The woman in the article has 5 healthy children. Why isn't that enough?