For some reason, and it may be cost, as far back as I can remember, airlines operating smaller aircraft have always been reluctant to advertise, preferring some other promotional technique such as personal appraoches to travel agents.
In the late 90's Brymon were happily and profitably operating their BHX-LGW shuttle. They had to use a larger aircraft (DHC6 - SD3-30) but never advertised. Passenger numbers failed to grow thus route was closed down months later. Remember Community Express? No adverts and soon no business.
My last airline took over a route to Aberdeen from BMA. No advertising and after a month I distinctly heard a travel agent in the departure terminal building say that there were no longer any flights from that airport to Aberdeen. The enquiring passengers would have to travel about 50 miles to get a flight and we were doing 3 rotations a day!
P.P.