Apple has their standards high (though I agree they sometimes can't meet it themselves).

They wouldn't want their brand to be associated with some half baked feature that either runs very slow, or draws too much power/heats up, or unreliable.

So even if another version is technically capable of that feature, if it runs slowly/inefficient, they'd not enable it to protect their brand value which makes sense: as a user if Apple announces a feature, I know it's more "done" compared to the competition like say Samsung/Android etc. and I trust that it's not half baked (which sometimes fails to meet the expectations, but still MUCH better than the competition)

The business reasons are just a plus _on top of_ these reasons.

If it was purely a business one, they would not have put it on the Apple Watch 9 which is no longer available. Putting it on the 9 doesn't do anything for sales.

There must have been some hardware difference.

The Series 9 has a better low-power background accelerometer than the Series 8, which is likely the reason.

What makes you so sure it’s not technical?

The Apple Watch 9 and Ultra 2 have a neural engine (GPU), which may be helpful if the sleep apnea detection algorithm uses a neural network. Those models also come with a third-generation heart rate sensor, although that part may not be required if the algorithm truly only relies on accelerometer data.

atleast due to processing/battery reasons.