Jippie - there are a lot more people living in cold, grey and wet northern Europe wanting to visit warm, sunny and dry southern Europe for a weekend - Sicily for a weekend is more appealing to a resident of Dusseldorf compared to the reverse.
Furthermore, northern Europe (UK, Netherlands, Germany) tend to have greater disposable income compared to other parts of Europe and can thus afford those weekend trips away.
All this implies that an LCC is, all other things being equal, often better off basing more (but not all) of its fleet in NW Europe, than elsewhere