DBMS (and DBAs) tend to be more conservative, so extensions and implementations diverge much more slowly than, say, js.
There's also the incoming business argument in favor of not diverging too far from baseline.
If I'm AWS/Azure/GCP trying to attract a customer from a competitor service, 'completely rewrite your app to be able to use us' isn't a compelling pitch.
MS SQL Server and Oracle have different incentives, but the cloud services would probably prefer portability and decreased maintenance / fork support load.
There's also the incoming business argument in favor of not diverging too far from baseline.
If I'm AWS/Azure/GCP trying to attract a customer from a competitor service, 'completely rewrite your app to be able to use us' isn't a compelling pitch.
MS SQL Server and Oracle have different incentives, but the cloud services would probably prefer portability and decreased maintenance / fork support load.