Why did Shakespeare's use iambic pentameter for dialogue of noble characters while commoners often spoke in blank in verse or prose

We're led to believe that Shakespeare used iambic pentameter for the dialogue of noble characters sense they were fully literate and commanded a more elegant way of speech, or at least that's what we draw from the times. A commoner would speak in black verse/prose since they aren't fully literate and don't have as good of a command of language. It was also a way to pick out those of lesser stance while reading/watching the play, so the audience would know what their place in society is to understand the play better.