What do you mean by "carrier exception"? Exception from policies those same carriers set up decades ago?
It's not like phones where pSIMs were relatively brand agnostic, were back to per-hardware BS.
I can imagine Apple being less inclined to allow arbitrary eSIMs on their watches, preferring to only support the case where the eSIM is guaranteed to support the "correct" presentation.
How nice of them to make an exception.