It is difficult to imagine that a plant can grow at the same place for many thousands of years: Climate and local environment change. Different species grow up and die out. Nonetheless, some plants get established, but don't manage to spread into new areas, because the climate is at the edge of what they can tolerate. Great fen-sedge is just such a plant.
Plants that grow in and beside water have to be prepared for marked and rapid changes in their living conditions. They must be able to tolerate living under water without drowning, and getting totally dried out without whithering. Many swamp plants are well adapted to these kinds of changes.