The Pencil sends messages to sensors built into the iPad Pro. The iPad Pro sends messages back. Together they determine the precise position and even the angle at which the Pencil is held relative to the tablet.What's the reason why this won't work on all iPads?
Would an iOS 9 update fix that?
They'd sell 10x more if Apple Pencil worked with all iPads ....
Older iPads don't have those sensors built in. No software update is going to add hardware sensors to your iPad. To use the Pencil, you must have an iPad with those sensors built in. Currently only the iPad Pro has those sensors. Maybe Apple will see fit to include the sensors in smaller iPad models in the future. They haven't said. Meanwhile, no software update can enable the Pencil to work with other iPads.