dude, its because s/he put out for Aizen. There must be some other reason, but what??|
I don't think Luppi was stronger than the other Espadas (7,8,9,10).
It wasn't about Grimmjow's motivation
Grimm went to the living world when he wan't supposed to, so he was punished -- made to realize how weak they could make him, and how his life rests in their hands.