I found out I was pregnant back on January 15th, I had a miscarriage at 11 weeks, apparently it was an atopic ectopic pregnancy and the baby died inside me at 9 weeks, but my body started reacting 2 weeks later. Because it was "early" during the pregnancy my doctor gave me a pill to induce labor aka abortion to clean my uterus from the baby's body.
It was the most painful (physical and emotional) experience in my whole life and while I was at home suffering the pain and expulsing my baby in chunk of blood, I couldn't understand how there are women that willingly go thru that. I was devastated and my husband didn't know how to help me, but being there.
First, my respects for your baby. You're an angel mom now. Look for the term.
Now, about your question, some are fooled into thinking they have no chance but aborting the pregnancy, some are too irresponsible and think the baby will get in the way of their hedonistic lifestyle. Some are pure evil and want to murder their own offspring just for political points.