[rcar]

From the History section of the Wikipedia page:

Jukeboxes became popular in the 1940s and were often set to a predetermined level by the owner, so any record that was mastered louder than the others would stand out. Similarly, starting in the 1950s, producers would request louder 7-inch singles so that songs would stand out when auditioned by program directors for radio stations.[1] In particular, many Motown records pushed the limits of how loud records could be made; according to one of their engineers, they were "notorious for cutting some of the hottest 45s in the industry."[3] In the 1960s and 1970s, compilation albums of hits by multiple different artists became popular, and if artists and producers found their song was quieter than others on the compilation, they would insist that their song be remastered to be competitive.

[Moru]

And if we go further back we find that there was a sort of brightness war in orchestra tuning going on a long time ago. Think they standardised on 440 Hz by now?

[Tsiklon]

I remember watching a video by a music theorist on YouTube (12Tone) where he mentioned that laws were made to define what Concert A would be - https://www.youtube.com/watch?v=BzznBt8tVnI