Why does FM sound better than AM?

Back in 2005-ish a Clearchannel enginer, Littlejohn proposed narrowing bandwidth of Am stations to improve the signal to noise ratio. They implemented a 5 Khz bandwidth. Back in the 70s we were all blasting 12 kHz. While that's probably OK-ish for talk, it's dreadful for music. https://www.radioworld.com/columns-and-views/the-5-khz-am-re...

12 kHz AM on a good receiver sounds absolutely fantastic.

I have a Royal 51/810 (one of each) that I use as a travel/bathroom radio, ironically, both have fantastic AM performance, and.. lacking FM - the IF/Audio bandwidth appears twice as wide on AM, and FM just sounds like crunchy - probably needs caps in the audio section, but its so tightly packed, and has a PCB with the heaviest plating I've ever seen - which means it needs work I cannot easily do.