I'm glad they didn't write it in Objective-C, as this would probably make compilation on Linux much harder (I have no idea what the state of Linux support for Objective-C is). As for writing it in Swift itself, that's not possible until Swift is sufficiently stable and with enough library support. Which it may or may not have now, but certainly didn't have when they started ;)
Linux support for ObjC is great on the compiler side (GCC supports the full language), but none of the core libraries are available. You can get some stuff (NSString, NSDictionary and so on) through GNUstep, but last I checked it was an uphill battle to be truly productive with ObjC on Linux.
GCC doesn't support the full language anymore, since the stuff that Apple added after finishing the migration to Clang hasn't been ported to GCC (e.g. generics and array/dictionary literals) and there doesn't appear to be anyone interested in doing so.
