I would say a combination of popular SPD incumbent Johannes Rau (PM from 1978-98), lack of a decent CDU challenger and opposition bonus against the CDU/FDP federal government.
From what I've heard they ran very good election campaigns. The SPD successfully labelled itself THE NRW party, promoting social justice and government help for the declining coal and steel industries in the Ruhr area
From the 80s to the mid-90s the SPD generally did (much) better in state elections than in federal or local ones in NRW.
Very interesting. Thanks
