I see it a little differently than you guys...
Apple is heavily vested in IGZO. however the screens for the R Mini weren't ready yet.
I think this was pretty evident leading up to the R Mini's announcement. There were lots of rumours for months stating Apple was not likely to release a retina Ipad Mini till 2014.
Then started coming the competition, who were managing hi DPI displays in 7/8" tablets mid 2013. the Nexus 7 was a bit of a blow to Apple's plans.
the Average consumer doesn't give a flying fruitbat what type of technology makes the display. They just care that the resolution is there, Viewing angles are sufficient and that colours are close enough to not appear washed otu and bad.
In this case, With the N7's release, Google camp was ahead of the game.
I think Apple did a very knee jerk reaction to this and sourced the first available Retina Mini panels they could, within a certain price range, that matched a minimum requirement of at least decent enough colours.
Apple was not going to give google and the other makers of high resolution tablets 6 months headstart.
Hence, we get a Retina Mini with a compromised display. Not the one that Apple intended. But still good enough to be sold.
What we'll likely see, is similar to what Apple did circa the iPad 3. a short lived product that was a stopgap product. in 6months or a year, the rMini will be once again "refreshed" and we'll likely see the IGZO displays that were likely expected to be released originally in 2014.
So overall, I think it's a combination of factors that lead to this display. Ands not just Apple trying to be cheap, though it was a consideration