[HamiltonJay]

That’s because not ever vendor puts its own stupid skin and hacks on top of the OS like Android... So, you will never see Android updates reliably and regularly released because every damn vendor has to recompile, test etc their code, and guess what? They aren’t software companies so they don’t waste their time, they just focus on wedging their skin on the latest version for their next phone... no money to be made trying to backport it to their older devices.

[RayChuang88]

Because Apple controls both the hardware and software side, they don't worry about third-party "skins" that can affect how the upgrade goes. Indeed, Apple has done a LOT of internal and third-party developer testing to make sure the current iOS 8.3 works with most newer iOS devices--iOS 8.3 works very well with the iPhone 5, 5S, and 5C in addition to the iPhone 6 and 6+. That's why the iPhone is so hugely popular now.