Ryanair's blind spot

I was looking at Ryanair’s destination map today, and it suddenly occurred to me that while they fly to lots of destinations all over Europe, and some areas are covered very densely, it seems they have a kind of black spot in an enormous area covering eastern France, southern Germany, Switzerland, some of Austria and the western part of the Czech Republic. Sure, they fly to Friedrichshafen and Karlsruhe, but only from very few airports.

Why is this? Are the airports too expensive in this area? Or are there too many competitors?