Why might Shakespeare have chosen blank verse for the dialogue spoken by aristocrats Romeo and Juliet?

In Romeo and Juliet, Shakespeare employs prose, blank verse, and rhymed iambic pentameter in the dialogue. He does this to aid with characterization, create mood, and add import to certain scenes. The audience would not have consciously paid attention to which sections of the play were written in which form of poetry or prose.; they would have been so caught up in the plot that the variety of written form would only affect them subconsciously. But Shakespeare certainly knew what he was doing.
For the most part, characters like servants and lesser members of the two warring houses speak in prose. At times, Romeo's companions speak in prose as well. This signals to the audience that these characters are not as well-bred and/or that the content of the scene is more mundane. These lines often move the plot along but don't bear the important thematic content of the play.
The important characters of the play—Lord and Lady Capulet, Montague, Romeo and Juliet, Friar Lawrence, and the Prince—routinely speak in blank verse. Their dialogue is structured in lines of ten syllables with every other syllable accented. This makes their speech sound more formal and well-considered. It helps establish their class in society, and it signals to viewers that the mood of these scenes is more serious. It helps the audience know that they should be paying attention to these words because the serious matters of the drama are being delivered.
When Shakespeare wants to add even more import to a scene, he adds rhyme to the iambic pentameter. Sometimes a speech will be delivered in blank verse, but the final two lines will rhyme. At other times, such as Romeo's speech that begins, "Why, such is love's transgression," an entire speech is delivered in rhymed couplets. Here Shakespeare signals to the audience that weighty matters are being declared, either for understanding the theme or for grasping a character's deeper motivations. This form of dialogue adds to the entertainment value of the play because the poetic language is so pleasing to the ear.
Shakespeare used iambic pentameter to indicate the social class of characters to the audience, but also to signal that the dialogue being delivered was important to fully comprehending and appreciating the drama.