Most likely, because you are trying to install 3 major updates, and/or because the wifi network speed is not so high.
Since the device is still running the prehistoric iOS 13 version, I suggest that you try to update that iPhone 8 via computer (if you don't have a computer borrow one from your friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider), then learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.
If you still get no help, contact Apple Support directly, using this link:--> Get Support
Regards
Giulio