ahhh yeah, maybe data analytics on the internet helped labels learn that songs without a key change had a better chance of going viral.

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.