Why did Tybalt attack and kill Mercutio instead of Romeo, who he had originally been targeting?

Though Tybalt's quarrel is with Romeo, he begins the momentous duel in Act 3 by fighting with and killing Mercutio instead. Tybalt ultimately attacks Mercutio because the garrulous and hot-headed character insults Tybalt and goads him into a duel to protect Romeo's honor.

From the beginning of the encounter in Act 3, Scene 1, it's clear that Mercutio is itching for a fight. For instance, when Tybalt signals that he wants to talk to Mercutio and Benvolio, Mercutio responds "And but one word with one of us?/ Couple it with something; make it a word and a blow" (38-9). Basically, Mercutio is egging Tybalt on in an effort to induce him to fight. Tybalt ignores Mercutio's attempts until Romeo joins the scene and refuses to fight. Upset with Romeo's pacifism, Mercutio unleashes a heap of insults on Tybalt, calling him "rat-catcher" (73) and "Good King of Cats" (75) and saying "Will you pluck your/ sword out of his pilcher by the ears? Make/ haste, lest mine be about your ears ere it be out" (77-9). In short, Mercutio deliberately goads Tybalt into a duel by insulting him venomously. It is not until Tybalt kills Mercutio that Romeo is driven to draw his own sword.