The title of the play has caused some comment in the past because Antonio does not dominate the action or claim most of the audience's attention. Shylock is really the central character in this regard until his exit at a relatively early stage, while Portia is the dominant character - or at least jointly so with Shylock - during the trial and from there to the end. Antonio is curiously muted and passive for an eponymous character and his opening words really characterise the way he is to behave for the rest of the play.
A 'Merchant' is an old word for 'Businessman'. This play is about the struggle between a Jewish banker (Shylock) and a Christian merchant (Antionio) . They both live in Venice.
So The Merchant of Venice is called that because, basically, it is about a merchant from Venice!