ios 12 to 13

Because it’s hardware is not compatible with anything newer than iOS 12.

BTW, anything that can run iOS 13 can also run iOS 14.4.1. So nobody is being offered the option to update to iOS 13 any longer. iOS 13 is effectively no longer available to anyone. Any iOS 14 capable device running iOS 12 would only ever see the option to update to the latest iOS 14.4.1.

As I said, no. An iPhone 6 cannot be updated beyond iOS 12. Its hardware is not compatible with any newer version of iOS.

Because the iPhone 6 does not have the hardware to run the newer iOS versions.