T211,
Getting slots at the right time of day into DUS maybe the issue and at the face of it DUS should be a winner due to the lack of Ryanair city size, industry etc. Hard to know the specifics unless you are on the inside. EasyJet from memory only had a daily evening service which certainly doesn't fit to their business traveller agenda, so one could surmise it is slots and commitments to the morning slots at Gatwick.
You prob already know that you can get BA, Eurowings /LH ex LHR... May not be attractive though.
EI -BUD