It's because the other countries don't have the same kind of system. You can only strike at the end of a collective agreement regimen. The other countries have an entirely different system. North America is unique in the world in how it's done. In other countries, people are free to join whatever union they want, but they're all in a union. There aren't very many non-union people in other countries.
On May 7th, 2013. See this statement in context.