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

This is not quite accurate. While iambic pentameter is indeed one of the most prominent meters in Shakespeare's works, it was the standard verse form for all dramatic poetry of the period, regardless of a character's social status. Both noble characters and commoners would speak in iambic pentameter, as this was the convention of the time for theatrical verse.