I agree with the post above.
I shall add ground conductivity to the list. If you're amid pine trees or other areas of low conductivity, it simply lowers your coverage.
The sunrise/sunset times affect even stations in the southern states. When sunrise is 7:30 in October and March, you've lost half the drivetime AM hours. Likewise, when sunset is 5:30, you lose that driving home crowd.
Consider the shortest days are within the busiest shopping period of the year, so you have the fewest commercial avails when everybody is buying.
In small markets AM radio can still be quite a success but I boil this down to a couple of factors: fewer radio choices for listeners and programming decisions stations make.
In fact, I was on the road looking for any station to listen to and caught the AM in Pecos, Texas. The station is country and lots of local programming in the AM hours (news, weather, swap shop, farm reports, etc.). I was captivated as I tuned in at 9 am to hear the legal ID followed by 20 minutes worth of 30 second units played back to back. That is not a typo. At 9:20 they began the Swap Shop that lasted until 10:15. At 10:15 was a 15 minute paid devotional from a local Church. From 10:30 to 10:57 was a 5 minute network newscast, weather and a farm report (I think around 3 to 5 minutes). The remainder was commercials. They wound out the 10 am hour with a country song before network news aired at 11 when I lost the signal. Pecos is about 8,000 people. Oil is big there. Just try to find a motel room. The highways are full of oil field traffic 24/7. The roads are not as busy in a major city at 10 at night on a weeknight compared to my drive in to Pecos. The night scene, even 40 miles from Pecos looks like stars in the sky, dotted with lit up drilling sites in every direction you look.
As a footnote, many of the ads were for services that would be of interest to temporary residents (oil industry workers) and advertising jobs. I'd say that was half of what I heard. In fact there was a good deal of agency placed ads (McDonalds, Dairy Queen, Ace Hardware, etc., where a local franchisee might direct dollars or pressure agencies to make a buy for that market).
My point of all of this is when you run your station as a viable station, have a good sales staff that will make calls and establish relationships with local business owners, especially in a smaller market, the AM can do quite well. In fact, I simply was astounded by the number of commercials I heard.