Why do SPD have such strong position in the north east?
Traditionally strong for the SPD going back since forever (during the Weimar Republic it was generally better turf for the SPD than the Ruhr), and that would have been because it was a Protestant ('rural') industrial area; the dominant industry was textiles. Protestantism in parts of that area (Lippe, anyway) is also non-Lutheran, which may have been a factor early on or something (or so someone claimed in a journal article that I half-read about two years ago while looking for something else).
Thanks, I decided to look on the historical maps, and in
rural areas it seem that voting pattern follow the pre-1789 borders. The former secular principalities (which was mostly protestant) tend to vote SPD, while the former ecclessial principalities tend to vote CDU. The County of Lippe was Calvinist by the way, while Mindens and Ravensberg (the two other principalities in the corner) was was both Calvinist and Lutheran to my knowledge (as part of Brandenburg), but with a strong Lutheran dominance on the ground.
I must admit that the fact that 140 year after the unification of Germany and almost 200 years after the complete unification of Nordrhine-Westphalen, the old religious patterns are still so important for how people vote.