I imagine:
The idea of rituals has a lot of good tone for the game, and they needed a rule to ensure rituals had a good place in the system(--there is a direct relationship between system and tone or mood).
They set a mark that had some feel to it (tenth magnitude). It also ensured the most powerful effects would be reserved for rituals, suggesting the Order's basis in the Cult of Mercury. They didn't want everything to be a ritual, though, and certainly didn't want the SG to have to arbitrate every spell.