I think that has more to do with on the contour map WBAL-TV has strong penetration of all but 4 MD Counties and one Delaware County, those five excluded counties are the only ones that are included in the Salisbury DMA. WCAU only covers two northern MD counties SNF one Delaware County. The Salisbury DMA only accounts for Delaware and Maryland, leaving out the southern tip which is part of the Newport News VA DMA on the western shore.
All of this gives me reason to believe that there is significant coverage but lack of the need of a permanent affiliate for NBC since NBC can be received OTA on the for more than half of Delmarva peninsula save for the Salisubury market which receives WBAL via cable and a satellite. All of this plus the fact that Salisbury DMA is in a tourist area part of the state where there aren't many permanent residents.
WBAL-TV Contour Map: https://stations.fcc.gov/station-profile/wbal-tv/contour-maps/
WBOC-TV Contour Map: https://stations.fcc.gov/station-profile/wboc-tv/contour-maps/
WCAU-TV Contour Map: https://stations.fcc.gov/station-profile/wcau/contour-maps/