why did Brabantio assume that the Duke will side with him over Othello?

Brabantio assumes that the Duke will automatically side with him over Othello's "seduction" of his daughter. Othello is a moor and therefor considered inferior among the regular social hierarchy. Brabantio will claim that Othello has used witchcraft to seduce his Desdemona. Othello, however, is not your normal moor. He wins battles for the Venetians and is more valuable than Brabantio's quarrel.