Ryanair will be operating 5 flights per week from Teesside over the summer. Assuming pax turn up at the airport max 2 hours before departure, and they have to be ready for boarding 30 mins before departure, that leaves no more than about 1 hour dwell time in the airport.
The result is long quiet periods during the day with a 1 hour sudden surge in demand for food / drink. Making coffee or even tea is quite time consuming and labour intensive. Most people are not willing to work a 2 hour shift at near minimum wage. This is not a great business model for a cafe - they prefer to try to spread the demand over the day instead of all the customers coming when staff are potentially stretched to capacity
I wouldn't get too excited about the additional revenue to the airport from food/drink sold to Ryanair pax. Focus should be on car parking instead