He did it to show the difference in their status and education.
Iambic pentameter is an elevated style, and Shakespeare created noble and educated characters who spoke in it. On the other hand, there were commoners who obviously did not belong to the same class as the nobles and did not receive the same education (if they had any education to begin with) so he wanted to show those differences through their speech. Nobles spoke in iambic pentameters, and commoners in blank verse or prose.
