Up up up!
I think it may be in part because of this:
https://forum.atlas-games.com/t/a-problem-with-spell-containers/5660/1
Even with a spell, as it is written in the RAW (or at least, as I understand the way it works), if you cast while relaxed, nothing stops you from stockpiling non-ritual watching wards effects cast at the limit of your ability, potentially with the same trigger.
This also lessens the use of Watching Ward: A magus can trap his sanctum with spells, renewed each moon by casting without stress. So maybe the author wanted to go against that.
I think this is too bad, because, IMO, the ability to stock some effects, especially ritually sponted effects, in a limited duration watching ward, is awesome and reminiscent of a long tradition of magical characters (From Merlin in the Amber Chronicles to the Dying Earth wizards) and would have loved to do it for a few spells.