Possibly with BHD it is a case of it's not profitable as an ejet operation, if and when it swaps back to a Q400 it may well be but I know the route was losing money as an ejet route.
Using the 180,000 pax per year per airframe figure you quote, if you are running for arguments sake a 180 seat aircraft then that aircraft would only run 3 x per week at those loads to carry those passengers per year. Is that enough to attract carriers? I don't know because clearly that's 10-15 routes in total. Other airlines will look at route specific figures rather than a collective so that'll bring the figures per route crashing down which doesn't make a route viable for such a large aircraft, however, they may cherry pick the best ones and run them a couple of times per week, we can only hope.