Nobles Parma, but I dont think it's due to any specific guideline. In general I can't at this moment recall any reasons for having a non-Creo ritual of a spell below level 50 but for one reason: the feel of it.
The level 50 mark is only a guiding number and it is left up to the SG and troupe to decide finally on what to do as rituals and what not to. For further inspiration I've dug out this thread: [url]https://forum.atlas-games.com/t/why-is-above-5oth-a-ritual/2195/7]