Why iOS 13 is not updated iPhone 6 beta version

Keep in mind that you are talking about pretty old devices as far as tech goes.

The iPhone 6 was released in 2014, making it 8 years old now.

The iPhone 6s was released the next year in 2015, so that one is 7 years old.

The iPhone 6s has received updates for 7 years. Which isn't bad. My guess is that there will still be a few 'critical' updates here and there (such as 15.7.1 etc, etc) and iOS 15 support among app developers won't go away for a could of years yet. Which means you're potentially getting nearly a decade of use from this model phone..

The iPhone 6 cannot be updated beyond iOS 12.5.5.5 because it lacks the components to run newer software.

The 6 can only update to iOS 12.5.6

The 6s can be updated to 15.7 (which was THE “latest and greatest” iOS right up ‘til last week)