the loss of parking revenue
All ancillary revenue is vital for airports.
Faced with a choice of paying for fuel and parking,
I have seen cars parked on the outskirts of Shannon waiting for the phone call so they can drive up to arrivals. They wont even spend the couple of euro required in this scenario.
Unless there is a major road ie M18/N18 is Shannon's case and the N17 in Knock's there is not sufficient passengers using Bus Eireann for them to make the service viable.