Then why is it that other Apple products (both larger AND smaller) have retina displays, some for multiple generations? No way this is a issue about the tech not being there. This has EVERYTHING to do with creating space between the mini and full size iPad. Once the full size iPad has some killer upgraded features, the retina tech will magically arrive for the mini.
The tech doesn't exist in an iPad mini size currently without sigificant sacrifices that would undermine the purpose of the ipad mini.
The iPhone & iPod Touch have retina displays, but ones which are vastly smaller to the 2048 x 1536 needed for the ipad Mini.
The power required to drive a display of that resolution clearly requires more space / dimensions as evident by the iPad 3 & 4's increased weight over the iPad 2.
Likewise the iPad 3 & 4's battery is more than double the size of the iPad mini's and yet because of the power needed to power the more powerful device means it achieves actually less battery life than the minis.
Remember the iPad 3 & 4 requires the A5X, A6X a more powerful processor than the ipad 2 or the iPhone 4s (A5) wth its 980x640 resolution or the 5 (A6) with its 1136 x 640 resolutions. In fact 1136 x 640 in the iphone 5 is 727k pixels which is significantly lower than the 1024 x 768 resolution in the ipad mini and ipad 2 which is 786k pixels.
This is why the ipad 3 and 4 required the X abridged processor to handle the massive difference in resolution 2048x1536 = 3.1 million pixels which is a massive amount more than in the currently available 4" iphone 5 and ipad mini.
These processors require more power, bigger batteries and yet you want it in the 7.9" mini.
I'm sure apple could engineer it into the mini - but it would be thicker than 7.9mm and weigh more, and chances have battery life half of the current mini.
Add to the fact no-one is actually manufacturing a 7.9" panel with 2048x1536 resolution at the moment.