It actually makes sense. Women I know who've put up their kids for adoption have lived through their lives with the "What if?" and "Did I do the right thing?" and "Is he better off?" and "If I'd kept her, by now she'd be..." questions.
Abortion is much more final. Adoption, women carry the knowledge there's a living person out there that they brought into the world, but are not part of their lives, and have to live with being a "mother without motherhood." From all I've seen, it's more difficult to deal with, long-term.