Rhymed iambic pentameter is a metric used in poems with a very strict set of rules and it does not fit how Shakespeare writes the lines of his characters.
A blank verse also has a set of rules, even though they don't rhyme.
Stage directions is something that the writer adds about the character and how he should be portrait or perceived. Also not included in Shakespeare's dialogues.
Prose, on the other hand, is a free style writing that doesn't demand any sort of specific binding rules and exhibits a natural flow of speech, which is perfect for the context.
