Why doesn't Bremen belong to Lower Saxony?

Bremen does not belong to Lower Saxony because they are two separate states that each have their own constitution, government, and administration. Historically, Bremen was a long-standing independent Hanseatic city with special privileges and trading rights. It was not until 1815 that Bremen became part of the Kingdom of Hanover, which was later incorporated into the Kingdom of Prussia. After World War II, Bremen was granted the status of a state and thus became independent from Lower Saxony. Despite some cultural and economic connections between Bremen and Lower Saxony, they are politically and administratively separate. This explains why Bremen does not belong to Lower Saxony.