I don't know why Apple just didn't use OLED in the first place for the iPads..
I am sure that there are a number of reasons why Apple did not go with OLED but they have not, and will not provide the definitive answer you are looking for so we can all skewer them.
Here are some reasons I could cite:
1) There are very few OLED manufactures able to produce suitable quality panels particularly in the needed sizes.
2) Manufacturing yields are lower compared to others (Mini-LED is TBD still I think on yield)
3) OLED production, at least as to Samsung, was was on screens smaller than iPads, primarily Galaxy and iPhone Pro/Pro Max, laptop screens of 15" and above, and TVs. Over time this can fill out better, but there really are not screens available in the iPad Pro 12.9 size
4) OLED burn in continues to be an issue and over a 2-3 year time there will be a noticeable dimming towards the center of the screens. So that issue will become another issue to bash on Apple for a defective product.
5) Apple did not create the iPad Pro 12 as a product primarily to be used in the absolute blackness to view TV, it is positioned as a creative tool primarily. Those are not used in the dark.
6) OLED will bloom when turned up. I sure see it on my OLED TV, not as bad but still there. Inherent to anything that emits bright light really
Always going to be those quick to complain, quick to point fingers, quick to hate, quick to blame, quick to claim moral superiority, quick to claim technology superiority, quick to claim 500 other things, real or imagined, that they hate, dislike and can never live with....
Looking forward to getting mine...hopefully this Wednesday.
I'm thinking this will enhance my photography workflow to the nth degree.
For those thinking about it, I think you will love it 95% of the time. The other 5% you fall into the use category where it is not right. No shame or issue in that. Same thing why we don't all drive Teslas, Corvetttes, Bugatti's and other vehicles - they don't fit our use case or budget.