MiniLED at iPad sizes didn’t exist either until apple worked with its suppliers and invested a lot of money to get there. It was a lot more work than the oled ramp-up.
I agree, plus OLED panels for 13" and 15.6" laptops were already widespread before the release of the MiniLED iPad Pro, and not that more expensive, for instance, the 15.6" HP Spectre with OLED panel is just $50 more than the LCD one.
Edit: Just checked the Hp Store, the LCD option is not even available anymore, all the panels are OLED now and included in the same price.