Industry seems to explain some of that effect. Population density doesn't explain it completely, either:

https://en.wikipedia.org/wiki/Demographics_of_Germany#/media...

Other parts of West Germany also sport lower population density (Lower Saxony in the North, Bavaria in the South).

It is absolutely astonishing how strongly human behavior (industry and metropolitan areas) influences local environmental conditions. I guess most cities may (be forced to) change radically.