It is indeed all about bandwidth and quality .... or rather the lack of need for high quality audio .
AM provides adequate quality within a comparitively narrow bandwidth.
Even NBFM requires far greater spectrum space than AM ...and the extra quality is superfluous to the required standard of audio reproduction.
There were studies years ago that indicated that a comparitively narrow but relatively high audio freq signal gave much better readability in poor conditions ....you can hear this in action any time there is a lady controller/pilot on air - their voices with a relatively high AF are much easier to read than their male counterparts.
For the same reason - many Amatuer bands DXepeditions often include a lady operator as she will make far more contacts than the men- and not just because she is a female.