From my point of view, and a lot of people I know who use Aircoach instead of Airlink, the main reasons are this

- Six euro is quite a lot for an eight year old express BUS that takes 30 minutes or so when I can get a luxury 04 Setra/08 Jonckheere coach for one euro extra.
- The return fare for Airlink is not marketed enough. It should be on the side of the bus, people presume there is no return fare, so it's 2X6 euro = 12 euro. Same as Aircoach
- Some people prefer drivers to load luggage onto the coach, no problem with Aircoach but at DB drivers are not going to do that
- Lugging a suitcase up a large flight of stairs is not desirable when you may have just been on a long flight

There are probbably others I cannot think of. But with a little bit of effort the airlink could be improved and lose less passengers. I would be interested to see what the commercial margins are on the Airlink, because they must be making a decent profit at six euro, if Aircoach are operating longer routes, which more expensive vehicles run on for just a euro extra per trip.