Unless I'm mistaken, stage manager requires running 5 apps simultaneously when iPads were previously limited to 1 or 2? That includes 5 render pipelines (way more pixels to push than exist on the physical display). There would be hardware load on pretty much every key piece of hardware (GPU, CPU, memory, caches, disk), and by a non trivial factor.

So why gate it on macOS?

Same reason. Rendering more apps at full screen in real time takes a ton of GPU. Adds ~5x the pixels, and 5x screen scale transforms+effect. All the optimizations for “that’s hidden”/“that’s background” go out the door.