With the standards of domestic air services in the US, I suppose the deciding factor is whether the airline believe they can get away with serving nothing at all!
But seriously, I guess if a particular flight is historically heavily patronized by business travellers or international connecting passengers, there is a good chance the meal service would be better.
For instance, I flew on AA Flight 100 (DFW-JFK) a couple of years ago. That was their only flight of the day from DFW to JFK, with many passengers connecting onto trans-Atlantic flights (including me). Although the flight departed DFW after lunchtime (which normally means a snack-only flight), we still get a full meal deal.