The A9 (like the A8 was) is reserved for the cutting edge smartphones and there's VERY limited production capacity for the A9 in 2015. Apple already started producing the A9 at Samsung, because they need absolutely everything Samsung can make for their iPhones.
It's not like there is a choice. You will always use your high-end SoCs where they make the most sense first. The A8X is a different SoC that is solely targetted for the bigger iPads, which also have a way larger battery then the iPhone ever will. The bigger iPads is where Apple can go all-out, but iPhone is where it actually matters for Apple. Their whole strategy is based on iPhone first.
The A8 at least technologically is good enough for the mini, don't get your hopes up. If Apple decides otherwise, then because they want to broaden the range of sold devices with new technology that only the A9/A9X will have. But this won't happen too soon, since Samsung can only produce a limited amount of SoCs this year, their wafer output is limited and so is the yields of 14nm in the first year in mass production.