Why is Thomas said to be too passive a martyre to be a tragic hero?

Thomas Becket is the Archbishop of Canterbury and former Chancellor of England. Historically, he stood up against Henry II's demands that the Church subsume its authority to Henry's secular power, and ultimately died for the cause. In the play, he is represented as an overly proud and sanctimonious man who nevertheless transcends his weakness to accept martyrdom as God's will. As Thomas explains in his Interlude sermon, a martyr is not merely one who dies for God, but rather one who allows himself to be "the instrument of God" (199). He argues that a martyr is not made by accident, but rather by God's will. I think Thomas passiveness might be enough to actually make him a tragic hero.