because Tybalt is unaware that Romeo has married a Capulet
Dramatic irony occurs when the audience knows a key detail that other characters on stage do not. The audience knows that Romeo has married Juliet, Tybalt's cousin. However, Romeo is the only one on stage that knows this in the scene. When Romeo states that the Capulet name is a "name I tender as dearly as my own", Tybalt and Mercutio are confused and don't understand what he is talking about.