Maybe because most readers are too young to be dealing with pregnancy. I mean when I was like 15 that was the last thing on my mind… 
And maybe also because it often happens that pregnancies are written so unrealistic that you just can’t read it. Especially if you have been through it… I mean not everybody gets sick during pregnancy, pukes all the time, feels bad, gets cravings, mood swings etc. And not every childbirth starts with water breaking in the middle of a busy street. And it most certainly does not end with a mother dying during birth, unlike poor MC’s mother in every other story… 
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