We want to choose which metal has lowest reduction potential (strongest reducing agent) Stannic oxide is oxidizing agent due to inert pair effect you know. We know copper has high reduction potential and gets easily reduced, it can't even generate Hydrogen gas from acids. Iron can. But in alumino-thermite process, aluminium is used to reduce iron itself. Thinking intuitively we can tell it is not very easy to reduce aluminium oxide since aluminium is strong reducing agent itself.