HUY-CPH has not exactly been a huge success so far - seems dependent on a very small corporate client base with relatively low appeal to the wider business and leisure community. Any business (not just airlines) will prefer a diverse customer base - all eggs in 1 basket etc...
Ryanair's STN-DTM is (or was) I think a 1x daily at lunchtime, while Easyjet offers a morning and evening flight on Mon-Fri. Dortmund and the surrounding area is not Germany's tourist hotspot, so which carrier do you think picks up all the business passengers ?