Why is The Merchant of Venice named for Antonio?

It's difficult to say with definite certainty why Shakespeare chose the name The Merchant of Venice, but it's possible to guess with reasonable accuracy. In general, it's reasonable to assume that The Merchant of Venice is named after Antonio (who is a merchant and is from Venice) because, in many ways, most of the play's plot revolves around him. Consider, for instance, that Portia and Bassanio's courtship relies upon Antonio's help, as the merchant needs to secure a loan to allow Bassanio to pursue the heiress of Belmont. Furthermore, Antonio's inability to repay this loan results in the dramatic conflict with Shylock. In that case, it appears as though most of the play's action depends upon and revolves around Antonio, and so naming the play after him makes sense.
It's very interesting, therefore, that the character that audiences usually find most memorable is not Antonio, but Shylock. Indeed, the marginalized, Jewish moneylender's oppressed existence, along with his arguably tragic downfall, proves to be more engaging, memorable, and sad than anything Antonio does in the play. Therefore, it's understandable that the title of the play is confusing, as it's Shylock, not Antonio, that we remember once the final curtain falls.