The current iPad mini was reportedly kept from having a retina display because of its smaller battery capacity. The higher resolution display found on the iPad 3 and iPad 4 both require an incredible amount of power to drive, as well as more powerful and power-hungry chips. In order to keep the battery life around ten hours, the battery was nearly doubled from the iPad 2 to the iPad 3. Such a battery would be impossible on the iPad mini as it currently exists. Since the iPad mini was made to satisfy users looking for a smaller device, Apple couldn’t make it thicker to include more battery.
But with time comes improvements in technology. A new screen process could help reduce both thickness and power requirements enough to make a retina display feasible without sacrificing battery life. Certainly such an improvement would be welcome: while the current display isn’t terrible, it isn’t anywhere near as nice as even the Nexus 7, much less the full retina iPad.