Why does my iphone6+ not get all iso updates
The iPhone 6 and 6+ no longer receive software updates and is stuck on iOS 12 as it was launched in 2014.
List of iPhones that are supported: (Please note iOS 13 Supports the same devices as iOS 14)
Because your iPhone 6+ cannot be upgraded past iOS 12.5.2. You would need a newer iPhone model to upgrade past that.