I think because on Windows those two very different operations are overloaded onto the same key combo, and there could be situations where in context it is ambiguous which one would be invoked.
though I still don't like how you can't copy and paste files with cmd+c/cmd+v in os x :)
I can. I don't think I'd ever actually do that other than to verify, though.
You can remap the Command key to the physical Ctrl key on your keyboard if you want; this makes some muscle-memory actions a bit more smooth across platforms.
In general -- and I'm not sure what the official / historical word from Apple is -- I've always thought that the Command key came about because Apple wanted a different key for commands that the OS could intercept, without stepping on any pre-existing application level shortcuts that used Control.
It might have been cleaner had they just kept Command reserved for shortcuts going to the OS (cut/copy/paste, open/launch, window manipulation, bossing applications around) and told developers to use Ctrl shortcuts within their applications. But I'm not sure if that distinction would have been meaningful to most users.
The Apple HIG on Modifier Keys doesn't give much in the way of rationale for Command versus Control, except to say:
> Because the Control key is already used by some of the universal access features as well as in Cocoa text fields where Emacs-style key bindings are often used, it should be used as a modifier key only when necessary.
I agree that this is somewhat weird (though I'm sure there's a reason/opinion behind it) but it has never been a huge problem for me though.