I'm looking for concrete answer for my 5 year old who keeps asking me this question. He asked after watching the TV show Evolve (episode on communication). He knows humans and primates shared a common ancestor so I guess it's natural to wonder why other primates don't talk like we do. He has supposed that they don't talk because they are "just animals". I explained that we humans really aren't much different than animals so that isn't a good reason. I've already explained that they do communicate, just not in the way we do. I think I confused him because he tells me that I didn't answer his question.
What would be a scientifically accurate way to answer this question? Being 5 he has a very black and white view of the world, to him you either talk or you don't. He's not having any of this 'non-verbal' communication stuff.