It's interesting to think about it. Do they not speak because they can't physically, or because they haven't 'tried enough'/'hadn't had the need' to develop speech, making physical change to accomodate complex vocal skills useless? I think it's the latter, because I would imagine them being able to come up with some form of a highly developed vocal speech. That they never really have might be because they never really needed it and/or they never were mentally capable of such (though I doubt that).

One of the larger factors in advanced speech patterns developing is distance, I think. Take for instance whales. They have a vastly complex system of sounds to take advantage of water carrying sound at vast distances. Apparently their vocabulary is exceptionally rich.

If you're a family linked community who pretty much stays at the same spot, you simply don't need a lot of words (ie sounds). Especially if you have no contact with other groups.

Apparently early humans and chimps had the same or similar traits, giving them a seemingly equal chance, except that humans clearly did more with it. They did things that brought along the need for more complex vocal systems, namely languages and dialects.

"What is mind, don't matter. What is matter, nevermind!" - Homer J. Simpson.