A confident voice can ask a sharp question at the wrong moment and close a room down. In a meetup, the cost is one nervous speaker and a room that goes quiet. In a company, the same habit compounds, meeting after meeting, until the same few people decide what the team is allowed to say out loud.
You see the pattern at conferences first. The people who are confident keep speaking. The people finding their voice hold back. Over time the room narrows. The same voices return, the same ideas circulate, and the different takes never get a hearing.