Why wouldn’t an open source project announce a change to the place their source is hosted on their own blog? Why would they want to sweep it under the rug?
Does Apple not have an internal department that handles this for all their teams? Seems kinda weird for a division of a company to even have to choose their host.
The Open-source projects generally do things separately - e.g. llvm, as you're otherwise requiring apple set up a new account system (blocking contributing on iCloud account would seem less great), and building up its own UI and infrastructure for a git interface.
Also given that GitHub is a somewhat universally understood host that people seem to like, and it has all that UI/development integration that people like it kind of makes sense to just use that. It also seems that having GitHub accounts is increasingly widely spread so contributors would not necessarily have to create yet another account with yet another service.
