iOS 18 availability on all iPhones

Why was IOS 18 not put on all I phones

Because all phones cannot meet the requirements that iOS 18 demands.

In general, you need an iPhone XR or higher model to run iOS 18. Older phones like the iPhone 6, iPhone 7, iPhone 8 cannot the meet the requirements for iOS 18.

Not all iPhones have the necessary hardware in order to run newer versions of iOS. iOS devices that can run iOS 18 go from the iPhone XS up.

Because all iPhones don’t have the required hardware for it to work with. This is true for every iOS (or OS for any other brand). All devices will never be infinitely capable of getting everything new that comes out.