I seem to remember a feasibility study by some organisation (MAG perhaps?) which found that purpose built rail connections are not viable unless an airport is handling c. 5 - 10mppa. There are clearly exceptions to that rule, such as NCL (metro), but for the most part bus services suffice when there is enough business. I use the 757 between Leeds railway station and LBA sometimes when I need to be at the airport, and it is by no means an express service. I reckon if it was non stop it wouldn't be viable. I also reckon that the type of passenger using DSA would rather take private hire cab or drive, due to checked in luggage.