Longtime lurker. Dusseldorf is well located in Germany's most populous state of Nordrhein-Westfalia. Would estimate around 20 million people live within 1 hour of the airport and it is well served by road and rail. Reasonable to assume the flight attracts people going to both places. Not to mention good onward connections to Germany and eastern Europe.
The flight has been going as long as I can remember. On the pax numbers it seems to be doing ok, nothing fancy but steady increases going by the last few reports posted on here.
Prices have come down slightly since being taken on by Germanwings though I guess it would take quite a surge in demand to get them to put anything more than a CRJ 900 at present. Thats what about 90 seats and the A319s they use out of other UK airports hold circa 130?