It's fear. It's hard to get someone to pick up that flag and carry it, especially in an industry that underpays people to begin with. In the case of QA, you are asking someone who has pretty much been told they are a minimum wage making replaceable part to risk everything to try and champion a union, it's a lot to ask. In addition to that, the video game industry tends not to be all that big, reputation can mean a lot when applying to other places.
Almost all the QA houses, whether internal or external here in the Montreal region utilize the on-call system, so you are always at risk to be put on call simply because a manager doesn't like you. Hell, I've seen clients ask for people to get booted off projects simply for crossing unspoken rules and the external QA company will say yes no questions asked. These are the same type of companies that buy competitors, put all the new employees on daily schedules (so people don't know if they are working the next day) and then strip them of their health insurance because they don't want their old employees "to get upset".
The system is broken, and if it wasn't so broken it might not need unions to fix it. Unfortunately, there is basically no way out except unions at this point.