Why is it that the French settled parts of Norh America that became Canada retained their culture while the parts that became America were completely assimilated?
The Royal Proclamation of 1763 assumed that francophones would be assimilated. When this didn’t happen, that is, when there was very little immigration to Quebec by anglophones, Governors Murray and Carleton realized it would be better to guarantee their loyalty by giving them rights to their language and religion, property law and civil law. The Quebec Act of 1774 guaranteed these rights. https://www.thecanadianencyclopedia.ca/en/article/quebec-act
As well, the main population of francophones was always in the Saint Lawrence Valley and in Acadie (Roughly New Brunswick). The huge area of land that comprised Quebec at the time of the Quebec Act was very sparsely settled.
Population in 1774 was probably around 100,000 of which 2000-4000 were Anglophones. Compare with 13 colonies population estimated to be 2.5 million, mostly anglophones. http://faculty.marianopolis.edu/c.belanger/QuebecHistory/readings/quepop.htm
Short answer: low population in most of the parts of North America that were claimed by the French, greatest concentration of population in the St Lawrence Valley, guarantee of language and cultural rights and freedoms.
Hopefully some real historians will answer your question.
There are areas of Canada where it isn't hard to find the descendants of French settlers who have been "completely" assimilated and there are areas of the US where it isn't hard to find the descendants of French settlers who haven't been "completely" assimilated. It's often a numbers game if, for example, you consider much of Quebec and south Louisiana as zones where people were least likely to be assimilated.