AM radio, like other frequencies in the low HF spectrum, travel using ground waves then ionospheric refraction during night which can let the receiver pick stations several thousands of Km away. FM and generally VHF frequencies require mostly the transmitter to be in line of sight. When a disaster occur, you can pick AM radio emergency transmission from behind a mountain or from much far away than a FM station. This allows the elimination of any dependency on repeaters which are a weak link, especially in less than free countries that censor radio transmissions, whose listener couldn't pick a station from another country if it needed a repeater in theirs. AM is for freedom, not for music.

In an emergency situation, you can build a CW transmitter off parts reclaimed from a broken PC power supply, connect a diy antenna made with a simple wire and be picked up from another country; a walkie talkie will stop at the 1st hill.