- With every iteration, OS's get more and more complicated, supporting new features, becoming more secure, and incorporating more and more API's.
- As you use a device, you load more and more apps onto the phone. More things need resources, no matter how little, it adds up.
- Users complain about performance, but also things like battery life. Sure Apple, or any phone manufacturer, can make a blazing fast, buttery smooth phone, but at the expense of battery life.
It's all a compromise, not a conspiracy.