Why do some radio stations still use amplitude instead of frequency modulation?
Because (as you note) all the receivers would have to be replaced.
Because international agreements allot that part of the spectrum to AM.
Because an FM transmitter with 5kHz modulation needs 25kHz channel spacing to avoid problems with adjacent channels.
Because an FM transmitter with 5kHz modulation only has an audio bandwidth of 3kHz - which is far less than AM (depending on the standard used 5khz, 10kHz or 15kHz.)