I wonder if this athiesm is a holdover from the time of the occupation of East Germany by Russia. People were afraid to say they were Christians. It’s only twenty years, after all.
Some undoubtedly is. But at the same time, it’s a golden opportunity for some church to offer the East Germans more than just secular success.
A better example might be others of the former eastern bloc. Poland, Hungary, the Czech Republic, etc. Yet they seem to be a lot more religious than the former East Germany. And yes, they do have a lot more atheists today, as well.