Also, Apple tends to figure out what they have and try to get it right before releasing it widely. They don't want to throw things out there and then cause a lot of forks and language wars. The idea that it should be open source from the beginning, or it's worthless, is kinda dogmatic. Apple knows that the most core technologies need to be open source, which is why Darwin is there, Webkit, etc. And which is why Apple's work on LLVM is open source, it's extensions to Objective-C and of course Swift.

Swift was always going to be open source, but Apple launches OS products differently-- it gets them to 1.0 so they are strong enough to stand on their own.