Different animals. AM can bounce around between the land, sea, and ionosphere for great distances. This is why the great "Clear Channel" stations like WBAP, or WOAI, or KMOX had space on either side of them after the Sun went down so that they had clean reception over great distances. In days past, we could pick up Cubs games from Chicago, or the Cardinals from St. Louis, depending on how the atmospherics were. Those bands are touchy, and sometimes surprising; It is said that Fuchida's low-power signal "Tora, Tora, Tora!" from the Japanese attack on Pearl Harbor, intended only for the Kido Butai just 200 miles away, was heard in Japanese waters where the Fleet operators were monitoring comms. The reflections off water and wave can sometimes be mysterious...
FM is a different bandwidth that does not reflect off land or atmosphere. It is line-of-sight only, and cannot broadcast over the horizon and is dimmed by ground clutter as distance increases. Despite these shortcomings, it's signal is very much clearer than AM, so much so that Steely Dan wrote a song about it. Because of the limitations of the medium, and it's different bandwidth from AM, broadcast wattage could be increased without "walking on" or, interfering with other stations via bleed-over onto neighboring stations on the dial.
AM has suffered through the years since FM came along. Stations still broadcast at the same wattage they were assigned in the 40s, and no effort as ever made to increase or modernize the bandwidth. Stations in foreign lands (See: Mexico) broadcast in many multiples of wattage allowed Domestic broadcasters. Additionally, at night, these foreign broadcasters do not power down as most American broadcasters (aside from the Clear Channel stations) do, so they "walk on" many stations on low-power, rendering them unlistenable. Whereas many years ago, one could fish out music or sports from far away at night, now the dial is a sea of static punctuated by brief bits of mariachi tunes...
And, trust me: Don't try the barbequed iguana.
