Face time is kind of superfluous when you could just use skype.
Not everyone is going to download and install skype.
Anyone using an IOS device will have facetime, without them needing to do anything else. And on an iphone, facetime is integrated into calling app, so if someone calls you, you can easily facetime them back on a whim without needing to exit the app.
Simplicity is a very strong selling point for IOS, and basic services like facetime and imessage showcase this best, IMO.