Why is iOS 13.0 not there

iOS 12.5 is the highest iOS version iPhone 6 is capable of running. It was released now over 6 years ago, is too under powered, no longer supported and incapable of running the newer generation iOS versions.

You'll need to buy a more modern iPhone to get the newer iOS.

Because that model doesn't have enough RAM to run 13 or above.

(189300)