Why did the iOS update not now seaport iPhones 13

Because iPhone 6 Plus is a nearly 8 year old iPhone, which lacks the hardware required to update past iOS 12.5.x. If you need the newer iOS versions, then you need to invest in a newer, more modern iPhone which has the hardware to support the software. Your phone simply does not and never will.

Because the 8 year old iPhone 6 hardware cannot run an iOS version newer than iOS 12.