It’s easy to jump on a new standard when you don’t have any exiting standard it might conflict with. “Tearing it all out and starting over” doesn’t cost you anything when you never had anything to tear out in the first place. What kind of enterprise email/calendaring solution did Apple have to convert when they signed up for the new “standard”?
I don't think Apple had one. People were using various third-party packages. I'm sure a lot of people had to migrate those.