There's only a finite amount of space in National Cemeteries. While it's understandable that families wish to be buried together, having them side by side in a national plot takes space from others that have served our country. Perhaps something like a plaque with a note where other members of family are wouldn't be out of order near headstones, or if people were okay with being buried vertically (They do this in Europe when space is a premium); but not side by side.