Why are there no frequency modulated broadcasts on the long/medium/short wave, but only on VHF? I'm sure there are many historical and political reasons (long/medium/short wave were already in use for AM when FM radio was developed, VHF still had lots of free bandwidth), but are there also technical reasons?
FM radio gives a better sound quality than AM. Common FM on VHF also uses a much broader bandwidth and interchannel spacing than MW AM, so using the same bandwidth per channel much fewer FM channels would fit in the MW band than AM channels. But narrowband FM can be used with similar interchannel spacing as is typical for AM, so this is no reason not to use frequency modulation.
So are there any technical reasons why narrowband FM broadcasting is not used on long/medium/short wave, and if so what are they? I'm assuming that even narrowband FM would give better audio quality than AM, am I wrong about that?
(disclaimer: maybe there have been some FM transmissions on MW that I'm not aware of, in that case please read this question as "why are there nearly no FM broadcasts on the L/M/SW ...")
(I'm asking out of historical interest. I'm not planning on setting up any FM on MW or something)