Nope, it's not just advertising costs, ever since the 1960's the voluble, articulate, hand wringing, noise abatement, safety conscious brigade of do-gooders has taken the narrow minded view of such things as motorcycles as the totem of folk who are anti social, anti authority and just about anti everything else. Rural motorcycle sports such as trials, scrambles and grass-track racing, which was once so popular with both participants and spectators has just about died out. Even the iconic TT races in the I.O.M. are barely mentioned by the BBC whereas they were once broadcast live ,and although MotoGP and the Superbike series is considerably less predictable than the boring parade of F1 , it gets very little coverage.
So not just cost but bias.
Trevor