Earlier quoted context omitted.
Swift was designed for coding user-facing applications, not internet utilities.
Swift was designed to do everything - one language to rule them all. To quote from Apple's Swift page: "Swift is a successor to the C and Objective-C languages." My guess would be that the webcrawler was started before Swift was allowed to escape from the Developer Tools enclave.
Surely you agree that languages designed with one purpose in mind (a la Go and networking/server development) can reasonably do better at that purpose than an explicitly general-purpose language?
Just to be clear, I prefer Swift to Go in general. However, between the two, I would personally prefer to use Go for the development of network utilities.