Ryanair were reluctant at first about entering the Sun routes, If I recall rightly it was Go who went there in a serious fashion when the other LOCO's were not going after this market.
And one of the reasons was that many of the sun routes were very seasonal and the challenge was what to do with the aircraft in the off season, this story hasnt changed,if you need oodles of aircraft to serve Spain, Italy, Greece, etc there will be surplus to requirements in winter. Clearly if Ryanair could enhance profitability by operating the whole fleet in winter they would.
But more than all of that it is a reflection that Ryanair are more suited to the leisure market, given that many routes are at less than daily frequency and so much business now is also to sunnier climes!
EI-BUD