I think that upgrade was largely driven by a significant event, i.e. the move from 30-pin interface to the lightening port.
ehhh, but they didn't really change the form factor did they? wasn't part of the whole purpose of the move from 30-pin to lightning to save space so they could cram more <whatever> in there and make it thinner? They didn't do that.... That leads me to to believe it was done as a method purely for sales.
Plus, they'll have the TouchID to introduce. My guess is that the approach was twofold: 1. there wasn't enough lead time when developing the iPad air to successfully implement the touchID AND/OR 2. They're purposely leaving it out so they'll have another bullet point on the next iPad spec sheet. Apple is notorious for this... It's my belief that they could have put a retina display on the first mini. (maybe not...) but didn't so they'd have something to put on the next one.... I've said it 100x on here... I'm even more accepting of this because I'd rather have it executed properly than have a feature that simply doesn't work.