The Western European political system mostly depends on the center-right and the center-left working together and making compromises to reduce the support for more radical parties on the other side. During the Cold War, social democrats were addressing the same issues as communists, but their policies were far more palatable to conservatives.

That approach failed around 2000. When the left was protesting against globalization, social democrats had people like Blair in charge, leaving no moderate options for those opposing globalization. And when the nationalist right started growing a bit later, the center-right similarly rejected them and the issues they raised.

You could say that the center-right and center-left parties fail, because they are no longer center-right or center-left. Instead, they are divided into ideological and moderate wings. The former want to address the same issues as the more radical parties on the same side, but they are unwilling to collaborate with the other side. The latter are all for collaboration, but they are unwilling to disrupt the status quo.