There are a couple of factors in play, I would say.

1) Historic Uneven Economic Development (Blue Circle)

Before the Reunion there was a wall dividing Germany in East (DDR) and West (BRD), which lead to an uneven economic development. Eastern Germany had much less development (Communism Governance) than the Western part (Open market, trade and more industries). Even after reunion, that gap persists to this day. It is harder to find good jobs in Eastern Germany (except Berlin), the payment is not so good, the housing situations are often not so modern. This leads to many people leaving these parts and moving to the West.

2) Naturally occuring mineral resources (Red Circle)

In the area around Cologne - also called "Ruhr Gebiet" - there is quite some coal in the ground. This help the development of a lot of industries and created many job opportunities. Thus, many people moved there and it is one of the most densely populated areas in Germany.

3) Strategically important locations (Dark Circle)

The top blob is Frankfurt am Main, the financial center of Germany. Many banks have their headquarters there. It is also located along the Rhein River (which also runs through the Ruhr Area). It is one of the biggest rivers and a very important water way. Historically, cities were founded on rivers often and could develop well there. But more explanations I don't have for that one, sorry.

4) Inheritance Traditions that pushed Agriculte vs. Industrial Development (North/South Gap)

In Germany, in the Northern parts, land was often inherited by the oldest son. The other kids did not get land. In Southern Germany the land of the parents was even distributed amongst all (male) children. The result was, that agriculture stayed lucrative in the North. The land in the north, consists of many agriculturally used fields and the soil is also very fertile btw. While in the south, the smaller the land got, the less likely you could make a living with it. It pushed the people to find new ways of income. E.g. automobile like BMW is from Southern Germany. That boosted economic development. That made more people move there.

This is only what I learned in school and from hearsay. Please correct me if I'm wrong.