That wasn’t the reason. The issue is that, due to the abnormal way King Arthur is being summoned for the Holy Grail Wars, she is only arriving with what was on her person when she was on her deathbed (her armour, and Excalibur). If she were being summoned from the Throne of Heroes like a normal Servant, she’d potentially be summonable as Lancer (she might have also fit Archer, Rider, or Assassin).

Maybe she could have been called as Lancer if someone found Rhongomyniad and used it as a summoning catalyst?

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