I think that's because Sonia really regrets her decision. I agree -Sonia did what she thought was best at the time but back then she was in a different place, clearly confused and in a difficult position. She was 15 and just given a birth to a baby she didn't plan for. She knew that she couldn't offer Chloe the life she deserved. There have been many mothers in similiar situations who have regreted thier choices months/years down the line. However Sonia made her decision and it best to let things lie now. It also depends on the families perspective on this and whether thier opinion of Chloe's best intrests will change over time. When Sonia heard about the death of Chloe's parents in the paper -her maternal instincts were telling her to check the welfare of her daughter. At that point her intention wasn't to get Chloe back but just to check that she was ok. However seeing Chloe is going to make it very difficult for Sonia to let her go and move on.

I don't think that adoption would be a option for her now but back then she thought she was doing the right thing. After kidnapping Chloe, only months later it became apparent that Sonia deeply regreted her decision and has done ever since.