Market forces: supply and demand etc.
If Cork wants services and therefore passengers, revenue and jobs - not just at the airport but within the local economy - they have to compete with all the other European airports who are desperate for Ryanair services.
The aircraft will be deployed on routes where the airport authority has a more enlightened perspective and can see the benefits of fiscal support.
Thatcherism in action
Another way to look at is that there is only one Cork airport, but LCC's are ten a penny. If Ryanair can't provide the service another airline will.