Because the iPhone 12 Pro, it is the direct replacement for the iPhone 11 Pro.
With the iPhone 12 Pro, there is no need for an iPhone 11 Pro. That's why the iPhone 11 Pro is no longer sold directly at Apple.
As an example, Ford introduces a new Ford Focus, then they do not continue to sell the old Ford Focus.
New comes out, old is sent away.
The iPhone 11 is the cheaper version of the iPhone 11 Pro, with some compromises, such as LED display, instead of OLED. Aluminum frame, instead of stainless steel. Slightly downgraded camera - but still a really good phone for the price, and is still a Top model with FaceID.
iPhone XR is from 2018, and came as a downgraded alternative to iPhone XS, and with aluminum frame, LED screen instead of OLED, but with FaceID - it is an even cheaper alternative, and in fact the cheapest alternative if you want a top iPhone with FaceID and big screen.
iPhone SE, or Special Edition. Is a tuned version in an older design. It comes with the latest processor for the time it is released. The first iPhone SE was released with the same processor as the iPhone 6S, and the current SE has the same processor as the iPhone 11 Pro. Most importantly, it is the cheapest iPhone on the market.
That way you hit wide so there is an iPhone for everyone.
