Why do you think that Bradbury would introduce Clarisse before Montag's wife, Mildred?

Although Clarrise's role is brief, she symbolizes some of the major themes that Montag aspires towards. Mildred is basically a character that is dead inside. Montag has abandoned all hope that her soul can be revived. Even though Clarisse is absent for most of the story, the idea of Clarrise resonates until the end.