Because Apple knows it's a pointless little gimmick fapped over by fanboys only, and those are going to buy the new iPads anyway. The iPhones need to be announced with some kind of gimmick to cater to wide-eyed people, but fortunatelly the iPads have been catering to more mature audiences. It was the same thing with Siri - Apple made a big splash announcing a (still broken to this day) feature for the iPhone that would not be available to the same year's iPad.
Wrong....
Im no Apple fanboy, and the only apple products Ive ever owned are the iphone 5 and 5S.
TouchID is the biggest feature for me making the jump, and now I dont think I can purchase another handheld Apple device without it.