why cant ios 15 download?

Per chance, do you have a beta profile installed on this iPhone? If so, it will have to be removed before you can download iOS 15.

Otherwise, your phone needs to have either a Wi-Fi connection to the Internet or be connected directly to a computer, running iTunes/Finder, to upgrade it.

If you participated in the iOS 15 public beta, you were offered to install the beta profile. You can check by going to:

• Settings > General > VPN & Device Management

Ref: How to uninstall iOS beta software - Apple Support