The simple truth is that the sound quality on AM radio is so bad that it's almost unlistenable for music for the average radio listener. Therefore, no matter what kind of music you put on AM, not many people will be willing to endure the totally shitty sound quality in order to listen to it. I'm sorry to be the bearer of such bad news, but reality is what it is.
Tell that to Greater Media... WMTR Morristown, NJ, in the New York City MSA. $1.5 million in billings last year on a fairly limited high-band AM signal in the shadow of the New York City stations. And a 3.5 share in the Morristown embedded market consisting of Morris County, NJ.
There are quite a few of those examples, mostly in similar fringe suburban situations or in smaller markets.