If I had to make a guess, it would be that there has been very little of tangible value offered with most of the monthly watch updates, making them expensive and risky to maintain for little offsetting benefit to the consumer.
While I want/demand monthly updates for my Pixel 8 Pro, I would/will be fine with fewer updates for my Pixel Watch 2.
There were also some issues with WearOS 5 that led to a pause in the rollout. I believe it's been corrected now. My Pixel Watch 2 just recently to the November patch.
I would also suspect that there are fewer system components in the watch that would need patches or fixes, and therefore a monthly update may not be absolutely necessary.