> reasons for why everyone seems to have moved away from 1 big SQL database
I'm sure there are going to be other answers for the code side of things, but for ops:
Depends a lot on the size of the service, but in some cases: We got enough data that 1 big SQL store makes ops hard. (Took me 3 days to drop a table recently in a way that wouldn't affect the users) And splitting data became easier than before with specialised backends. (A sharded 2nd layer cache of live data seems way simpler to achieve than say 2 decades ago)
I'm sure there are going to be other answers for the code side of things, but for ops:
Depends a lot on the size of the service, but in some cases: We got enough data that 1 big SQL store makes ops hard. (Took me 3 days to drop a table recently in a way that wouldn't affect the users) And splitting data became easier than before with specialised backends. (A sharded 2nd layer cache of live data seems way simpler to achieve than say 2 decades ago)