OLED does have issues, especially with static images like menu bars and docks. There are mitigation issues that handle general use, but folks like to post pictures of display model iPhones with OLEDs that have significant burn-in/image retention because they show the home screen a dozen hours a day for months on end. The average consumer likely keeps their iPads longer than their iPhones (even if they keep iPhones for years) so this new OLED tech Apple is looking at is said to offer longer usable lifetimes, which would be relevant to longer device ownership cycles.
iPhones use a pentile RGB OLED panel which are very different from the WOLED (and QD-OLED) panels used in televisions. I am not sure what sizes those are available in and how much they cost so it might be that they either were not available in iPad Pro and MacBook Pro screen sizes or they are, but would add hundreds of dollars to the BOM (and MSRP).