Gurman said the delay is due in part to Stage Manager, which, in our preview of iPadOS 16, The Verge’s David Pierce didn’t like very much:
We’ll reserve full judgment for our review this fall, but so far, I hate Stage Manager. The piles take up too much room on the screen, and it takes way too much work to place the app windows just so. (One funny beta moment: when you turn on Stage Manager, it instantly forces the Settings app to render at a size it doesn’t support, and it breaks. Lots of other apps do, too.) When you open a full-screen app, you’re out of Stage Manager, and it’s not obvious how to get back or put things into the piles.
Gurman also says that the shift will allow Apple to launch iPadOS 16 closer to when it releases new iPad hardware; he reports that the company is working on an iPad Pro with Apple’s new M2 chip and a base-level iPad that will finally make the switch to USB-C.