Our Fleet (Ryanair)

All their ground support is geared towards the 737 family. This reduces the cost and complexity of supply chains, as they need to keep track of a relatively small set of parts and equipment. It also reduces training (they don't need to train anyone on Airbus, or any other plane type). They only need 737 simulators and 737 pilots, making them interchangeable.

To move to, say, the similar A32x family would take an almost entirely new airline to be built from scratch. That's not going to happen unless their 737's start breaking apart mid-flight.