Why can’t I update my iPhone 6s+ From 15 to 17 or 16

The hardware and limited memory does not support the necessary changes the software introduced. In other words, it’s too slow. iOS 15.x.x is the end of the line. 😢

iPhone 6s was introduced in 2015. It’s 10 years old.

Because it doesn’t support it, due to hardware limitations. Your iPhone 6s Plus has been discontinued with iOS updates and cannot be updated past iOS 15.8.3. Consider looking into a newer iPhone that can run a newer iOS, ideally iOS 18.

Well, it is also very difficult.....(impossible).....to update an iPhone 6 to iOS 15. The iPhone 6 can't handle the requirements of iOS 15.

The iPhone 6s Plus cannot handle the requirements of iOS 16.

The iPhone 7 cannot handle the requirement of iOS 16 as well.

The iPhone 8 cannot handle the requirements of iOS 17.

I think you get the idea. Once your phone is 7-8 years old, it is time to start shopping for a new phone.