In the Vancouver area of British columbia, Canada, an important AM station recently went defunct: AM 730 Traffic. That was like the only reason to tune into AM anything around here while driving, unless you're into talk radio. Problem with that traffic station model is that you don't have a captive audience. Once the listener groks the traffic situation for their trip, they flip to another station (more likely than not FM, by the way) or their bluetooth or wired connection to their phone. That listener heard one or two ads at most (often none at all). People not driving have no reason to listen, either.

All the music stations encourage people to listen all day. "We are here for you while you work, or for your entire drive."