It's nothing to do with the military.
The primary reason for so many links to Düsseldorf is that it's the main industrial centre of Germany. Frankfurt is the financial centre. There aren't too many bankers that desperately need to move between Newcastle, Birmingham or Leeds and Frankfurt.
The second reason, and this is where Newquay and Inverness come in, is that it has an enormous catchment. Therefore it's a much better place to run leisure flights from than Frankfurt. For UK tourists it provides great access to Cologne and Rhine cruises.