Where's the profit? iTunes for Windows makes sense -- it's a free product which helps induce sales of a profitable product (the iPod). iChat for Windows would make sense -- it could spur sales of a Windows iSight.
But what's the profit in Safari for Windows? A simple web browser will not induce people to leave their current platform altogether. Did anyone use Internet Explorer for Mac and thus go and buy a Dell? I doubt it. Safari is good, but not so good that it'll bring back in sudden switchers the costs of porting it to Windows.
Better for Apple to port only those products that either make money in and of themselves, or which tie directly into devices that make money.
Safari For Windows