One of the big pro-family groups (I forget if it was FRC or NRTL) did a survey on this. Women in crisis pregancies want the crisis to be over: ie, to make a decision. The problem with adoption is, to women in a crisis pregnancy situation it feels like adoption just extends the crisis, because their baby will be "out there" and they will always wonder about the child. In contrast, abortion or deciding to keep a child feels very final. Many women in the survey said they would rather abort than put a child up for adoption, as they felt guilty for abandoning the child and worried the child might be abused. (Yes, I know it's dumb to think of killing a baby as the caring option, but this is what the survey found.)
The sad thing is it shows a lack of understanding of modern adoption options (such as open adoption) and has created an almost total inability for an infertile couple of moderate means to adopt (foreign adoptions and private adoptions are both very, very expensive.) Not to mention all the data showing a stable two parent home is far and away the best environment physically, emotionally, intellectually and economically for a child. Right now, only about 2 percent of women in a crisis pregnancy situation opt to put the baby up for adoption. The rest either abort or raise the child, can't remember the exact number but it was pretty much half and half.