I'm guessing because it has to insert code into main() to set up the coordination with Xcode.

EDIT:

Ok, I looked into it. There's no code inserted into main(). The way this works is Xcode sets up an internal breakpoint on _swift_runtime_on_report, which is an empty function that is called during fatal errors specifically to allow Xcode to break on it.

I don't know why this requires main() to be written in Swift.