Why? They need to keep their planes busy and they can either do that by growing the remaining bases or adding new ones. I'd say adding new ones is the prefered option if that means pocketing "marketing support", steahlty subsidies etc. One could argue that this is the at the core of Ryanair's business model and results in Ryanair moving across Europe like locusts. Whether or not Knock is the most likely choice for such a move is, of course, another question.