Reasons for having it in the iPhone 5S (2012):
-Apple has bought AuthenTec and has their patents
-Needs something to be special in the iPhone 5S
-Lots of rumors from both analysts and from news
-Something competition hasn't really done
Reasons for having it in the iPhone 6 (2013):
-All new design could allow Apple more room to implement it
-Haptic virtual home button with the scanner replaces physical home button
-More time for Apple to implement and get it right
-Could be used together with build-in NFC which iPhone 5S can't due to aluminum design
-Apple has bought AuthenTec and has their patents
-Needs something to be special in the iPhone 5S
-Lots of rumors from both analysts and from news
-Something competition hasn't really done
Reasons for having it in the iPhone 6 (2013):
-All new design could allow Apple more room to implement it
-Haptic virtual home button with the scanner replaces physical home button
-More time for Apple to implement and get it right
-Could be used together with build-in NFC which iPhone 5S can't due to aluminum design