Talking about adoption

I’d like to hear some advice from this group… I was adopted when i was an infant. I tracked down my biological parents about 10 years ago (biological father was already deceased, biological mother claims it was her friend and not her even tho all arrows and neon signs point to her). My biological father’s widow, however, has been quite helpful and has provided me with information, pictures, etc. She also regularly sends Christmas presents and birthday presents to my 2 kids. This brings me to my question… I’ve always told my kids I was adopted and explained in simple terms what I know. However, my 9 year old is now getting more curious about ‘Why Wouldn’t a Mommy and Daddy Want To Keep Their Baby?!?!?’ and I am just not quite sure what to say. I explain about different reasons people decide to adopt or hope to give a child a better life or whatever but he is old enough now to think in more complex ways about things. He wants to know why we just don’t go visit her? Or call her on the phone? etc. And since I truly DON’T understand how someone who gave a baby up for adoption would not be even a tiny bit curious about that baby I get kind of flustered trying to explain it all… Any advice?