he wants to act emo.. but his papa/creator (nomura) dont let him
Zexion isn't emo.
I think the most logical explanation is that he's either observant and saw no reason to speak, was shy/traumatized, or they just didn't want to hire another VA.
Personally, I think he was just being observant. Despite being a kid, Ienzo was willing to work with the experiments and encouraged Ansem to build a basement lab. I think he's still pretty manipulative as such.