I wonder if DUS was a more attractive destination than CGN. Surely it wouldn't make sense to serve both?
Düsseldorf has always been the "airport of choice" to serve the Ruhr, CGN being too far on the southern edge of the industrial region to serve it as effectively.
Attempts at BHX to serve both DUS and CGN have (always) ended in tears - with CGN losing out to the frequency and yield on DUS services.