Swift is still very young, constantly evolving, and simply not ready for big projects. Case in point: is there a single Apple app written in Swift yet?

This means that writing GitUp in Swift would be a ton of overhead for very little tangible benefit.

See examples of what well known Mac developers have to go through with Swift: http://inessential.com/swiftdiary. I'd rather spend my time on building the app rather than on dealing with Swift.