You have to understand the brief history of Germany since the WWII to grasp the full picture. In a nutshell, once East and West Germany were formed, that locked people in to essentially different systems. Once the wall fell, Germany was reunited. However, the East vs West dynamic in terms of poverty and voting, the West is largely paid more and has is liberal leaning, while the east side of Germany is still quite impoverish, so they vote for the AfD because they seem to be telling the people what they want to hear.

It’s essentially very similar to how the US is blue vs red. Cites are blue, while the rural parts are red. Germany is blue on the west, and red on the east due to Germany splitting and reunifying.