Request to be adopted as only child

We have 5 girls (2 still at home) and our youngest bio daughter is 17 and our adopted daughter in 7. I've inquired about 2 different teenage girls that are in foster care and available to be adopted. The girls I've asked about are 13 and 14. In both cases, they came back saying they want the girl to be placed in a home as an only child. Both time they didn't give much information as to why. One of the girls we really had our heart set on. (we learned as much as we could about her while our fostering license was moved from the county to a private adoption agency). Neither of these girls online bios said anything about needing to be an only child (I do see that from time to time with other kids). What are the possible reasons for being placed as an only child? Is it something we should pursue and try to convince them to consider our family? Except for being a danger to a younger child, I would think a family (with sisters) would be the ideal environment for a child that's been through the foster care system. (We have also had 8 long term foster placements, everything from infant, to sibling group of 4, to a troubled teen).