Norwich surprisingly has a pop of 300,000, however it is quite limited in terms of ability to attract enough people there to use an airport with only 440,000 using its airport last year.
Thats hardly a bad figure, given that (assuming all passengers both departed and arrived through NWI) 210,000 used the airport last year. That would equate to 74% of the population of Norwich. Of course I would assume that the catchment area is significantly bigger than 300,000 though, and then consider that STN isn't too far away!