sitigeltfel, very easy to explain.
Aircraft breakdown is the responsibility of the airline, while an ATC delay is not. As you know airlines need to compensate passengers for delays that are part of their responsibility. That's probably why Ryanair wants to make you believe it's a ATC delay (which means no responsibility, so no need for them to pay the EU compensation). Fleet maintenance is part of airline responsibility, so if that was the case, they probably wouldn't tell the passengers who then have the right to claim EU compensation
