Perhaps intellectually honest, but a weak conclusion nonetheless: in my understanding, getting Safari 5 to run on Windows requires porting parts of Apple's huge, complicated Cocoa framework to Windows. That framework is always changing, and so is the browser-- and they both primarily target OS X. With OS X, Windows, Cocoa, and Safari all moving quite a bit between Safari versions, I would be surprised if it didn't become harder to support Windows as the product grows.