It is common sense to accept that she is powerless. But she doesn't like being powerless and being forced to do something.
She likes to be in control all the time. And frankly, the more she learned about how hopeless her situation is, the more unwilling she became.
Sword Flash, on the other hand, didn't feel as resistant as she was. It was because he recognized that this was an opportunity for him to grow stronger.
He simply can't be as resistant as she is to being forced to protect Arthur since he has already enjoyed the benefits of his position as carerake.
It was because of the gate that he was able to make contact with the League of World Eaters, join them, advance to the sixth mortal coil, upgrade his beloved weapon, and take the leadership assessment.
He has gained too much to mind being made to protect Arthur. And since he doesn't even need to do much, he doesn't mind his situation at all.
