[quote=@kusanagifire]
Because she wasn't good enough for the Grail.
[/quote]

Indeed, that's very much the case. Since the Grail is the Devil, it saw that Seyrun was so no good that she was almost like him, of course Angra-kun doesn't want competition, so it let Seyrun go. Such a shame she has other plans in mind...

EDIT: Keep the arguments flowing folks, I can always expand her ego a couple extra notches. You don't imagine how much I like flaunting my characters.