Answer: Britain defeated France in the North Africa (so called Nouvelle France became English, its inhabitants had to accept English rule, so called "Canadiens" started to face danger of assimilation). End of Seven Years War. Many French speaking "Americans" migrated southward. Later on (in 1770s) so-called loyalists migrated from 13 province northward to Québec which means that number of English-speaking population grew significantly.
Explanation: England and English started dominating North America.