Just a note: it's illegal to fire someone for trying to organize a union. Not that it doesn't happen, but you often hear about how those people get their jobs reinstated, etc.
The point is that forming a union is a bumpy ride, even when the majority of employees see a need. Management will work hard to stop it and employees have to do a lot of work to make it happen. Unions won't organize at places where people are generally happy because the chances of success are low.