I assume because the iPad Air is in direct competition with the iPad Pro, that’s why Apple has refrained from giving it OLED sooner. It will eventually get a single panel OLED like the iPad mini is rumored to be getting later this year, because most of the competition is using OLED now so it’s inevitable. The iPad mini will get it first probably because the iPhone Fold is coming out this year and will be very similar to the iPad mini in size. So it makes sense that Apple would give it OLED first. Again, these are all just rumors, but it seems it’s the direction Apple is moving in.