Adoption isn't really an alternative for abortion. I will probably have to adopt when I am older and I wouldn't want it on my conscious that the child I adopted was because some woman was forced to carry to term. Not to mention all the health concerns I would have about the child such as whether or not the mother used drugs during the pregnancy or if the child would be born with a treatable disorder.
Another issue is the cost. Some states will help pay for the cost of childcare for low income women, but what if the women don't qualify? It's recommended that women have checkups, prenatal vitamins (some places do give them out for free, but I think those programs were now replaced with the diabetic drug program in my area), delivering, time off if it was a risky delivery or pregnancy, and more.
A second issue is what if the mother does not want the child to ever find her? Some women are very concerned about the laws changing to allow the children to find them (some places no longer have anonymous sperm donation so adoption isn't far off). Some people don't want the risk of having an adult crying on their steps years later demanding to know why. Especially if the baby was a rape baby.
Some people feel it is not accepting consequences if they adopt out the baby more than if they had an abortion. They want to be involved in the child's life. There is open adoption, but some States (not sure about the rest of the world as adoption law is tricky enough here) do not recognize open adoption as a legal, binding adoption. What they do is just see that the adoptive parents legally adopted the child so the birth mother has no rights.
Then, on the other hand, the mother may not want to get attached to the baby because she knows she can't afford to raise the baby but she knows she can't give it up when the time comes. As cold as it may sound, love doesn't buy diapers, cribs, clothing, car seats, food, bottles, vaccinations, well-check ups, toys, and more. They also may not want to rely on handouts.
Adoption is a hell of a lot more complicated than what people are thinking.
It is damn near impossible to get abortion stats since those are self-reported. However, usually more women who are past their teens get the majority of abortions. They also have children at home.
From here:
http://www.womenscenter.com/abortion_stats.html
(I can dig up the email to find out where he had his sources if people want it.)
Teenagers obtain only 17%. Women who are 20-24 years old obtain 33% of abortions. Interesting to consider so these are women who are typically in college or starting careers.
About 60% are obtained by women who have 1 or more children.
Also, did you consider the stigma of adoption? It's kinda hard to carry to 9 months and have no one notice. At least with abortion, no one will find out in the work or friends if you don't tell them. People judge in almost all circumstances since they clearly know someone else's life better.