

Hmm always thought it’s cause big cities need big fresh water, that’s why there’s a good chance that historically they formed near streams/lakes. Typically near the coast rivers slow down and branch out, giving good access to natural resources and also trade. Trade increases prosperity and exposure to other cultures.
Please note that in Germany you get 2 votes in the federal election. 1st is for a candidate to directly represent your district, 2nd is for a party nation wide. The map only shows the election result for the 2nd vote.
Here’s another map to show the party affiliation of winners of the 1st vote:
Colors are the same, except blue. Blue represents CSU, essentially the Bavarian version of the CDU (Christian Democratic Union).