You Don't "tell them" that!
My daughter was abandoned at an orphanage about a week after birth. After that she had NO parent until age two. Just 98 other babies and a few caregivers around her. My daughter could have been allowed to believe that neither of her bioparents "wanted" her. (BTW - adoptive families don't use the word "real" to describe bioparents, because adoptive parents are very "real". Might as well know about accepted adoption terminology if you aspire to become a participant in the adoption process)
But we tell her that it has nothing to do with HER. That she is WONDERFUL and that the choice her bioparent made to not be her parent is a choice we don't really understand or know the WHY about. There are many possible reasons, but we don't have answers. And I NEVER, EVER say a BAD thing about her bioparents!
We tell her that we DON'T know whether they "wanted" to be a parent to her: only that for some reason they knew they couldn't be the parent that she needed.