Not everyone wants to speak up, even when they have something to say. Some find more comfort in listening and observing. What do you think draws people toward that role? Do you feel more at ease talking or listening in conversations?
There are actually different reasons to this though, for some they simply feel no matter what they say, they would never be understood, hence no need talking, while for some that's just their nature, they don't want to stand out but blend in.