It's social status, and willingness/power/influence to talk to the press and be active in the search in front of the media (family members). Yes, wives and children are murdered every day and these often takes a small corner of the paper- those with the influence and "power" in their community, get in front of microphones and cameras, organize community searches, fundraisers, candlelight memorials, etc.
Not so in communities/areas where relations don't want the media and public to know their buisness, because not only is it murder, but there are past criminal records, past bad police reports, restraining orders, physical abuse reports, drug run-ins, kids in trouble- a messy family and they just don't want their family laundry aired out to dry in front of all to see.
Same with kidnapping little girls- white affluent families that can afford to be pushing their power and influence around and besides, if they didn't, they would look suspicious (look at the little girl, uh, unsolved... forget right now-the Ramsey case). Had they not been in the public's view, it would have looked bad for them.