Unity has a lot of old stuff lying around (they only very recently dropped DirectX7), so I'm not really shocked, but I was surprised it was still i386 because the upgrade should (well, okay, "should") be pretty trivial and i386 Macs have been out of support for a while.
My understanding is that the KSP devs have experienced instability and bugs with the x64 builds for Windows/OSX which they attribute to Unity.
In my experience (a few months out of date now) the x64 builds for Linux are not terribly stable, but perhaps the devs think they are more stable, or think that linux users are better equipped to deal with questionable stability and released that version despite the stability issues.
Unity has a lot of old stuff lying around (they only very recently dropped DirectX7), so I'm not really shocked, but I was surprised it was still i386 because the upgrade should (well, okay, "should") be pretty trivial and i386 Macs have been out of support for a while.