Why can't I update my iPhone 5 to iOS 11

Because the iPhone 5 cannot run iOS 11. It does not have the resources, ram, and CPU, to support iOS 11.

In short its not compatible with iOS 11.

The only way you can update is to buy an iPhone 5s or later. The CPU in your iPhone does not support iOS 11

The iPhone 5 does not have sufficient hardware to run iOS 11.