Why is the play The Merchant of Venice named after Antonio?

Many wonder about the title, since Shylock stands out as the play’s most dynamic character. He is crafty and cruel but sympathetic due to the antisemitism with which he contends. Second to him is Portia, the beautiful, wealthy, and clever woman who saves Antonio from Shylock’s wrath. In addition, much of the plot revolves around Bassanio, whom both Antonio and Portia love.
Still, Antonio is an essential part of the story. He funds the young nobleman Bassanio, even if it requires borrowing money from his enemy Shylock. Because of this act, Bassanio and Portia are able to meet and marry, and Antonio’s life is put in danger. The plot’s most tense scene involves Antonio’s imminent death at Shylock’s hands.
The word “merchant” is also key. Antonio makes his fortune off of selling and buying, and the play revolves around money and exchange. Antonio promises Shylock a pound of flesh for three thousand ducats. Shylock’s daughter Jessica gives both herself and her father’s jewelry to the Christian Lorenzo. Portia can deliver beauty and money to whomever she marries. Antonio gives money to Bassanio and receives Bassanio’s gratitude in return.
Another reason why the play might be named after Antonio is his ostensibly laudable qualities. This is how Bassanio describes him:

The dearest friend to me, the kindest man,The best-condition'd and unwearied spiritIn doing courtesies, and one in whomThe ancient Roman honour more appearsThan any that draws breath in Italy.

Taking into consideration how poorly he treats Shylock, it is debatable whether Antonio lives up to that high praise. If the description were true, he would truly be a character worth naming a play after.
http://shakespeare.mit.edu/merchant/full.html