NRW used to vote strongly on the left of the country between late 1960s and late 1990s. Not in the early years of the FRG. Maybe because of Konrad Adenauer.
Yeah I think it was the Adenauer affect in part, I do wonder if secularization of the state may explain something as well (I'm not too familiar with the religious demographic history of Germany so I can't say for sure one way or the other).
Then NRW has the Ruhr region that's very industrial and leftwing, along with having a big, fairly cosmopolitan city in Cologne so the CDU would really have needed to run up the numbers elsewhere in order to win.