I'm sure public transport is not high priority for DSA management. Maybe for workers and spotters, but the bulk of people passing through DSA (and the other regional airports) use cars or taxis. Public transport will serve the minority rather than the majority. If you've had the torture of using the Trans Pennine Express to MAN, you will appreciate that the masses offload in the City leaving small numbers to disembark at MAN - an airport handing 20x as many passengers as DSA.