There was a short period when iPhone users could choose if they want to stay on the iOS 14 version, or if they want to move to the 15 version.
If you weren’t interested in iOS 15 you could choose to stay with iOS 14 and you would get all security updates that were created.
This time however has paseed and Apple has officialy stated that they won’t be sending any more security updates for the iOS 14.
The reason it took so much time, was because iOS 15 wasn’t as stable as the company wanted. This has changed right now and you can download the iOS 15 without any problems.