I would guess a consolidation of labels with a few high output producers who were able to copy/paste hits and use MTV and internet to build a mainstream cookiecutter conveyor belt from teenage disposable income to their pockets. Along with 'billboard hot 100' being captured by same, and influenced to promote only the inside tracks.
ahhh yeah, maybe data analytics on the internet helped labels learn that songs without a key change had a better chance of going viral.
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