In summary: who is to blame?
There is not one single culprit, but an accumulation of divergent views on federalism:
Quebec's view: Two nations, asymmetry, protection of distinctiveness.
View of the rest of Canada (especially Trudeau and many anglophones): One bilingual nation, equality of provinces, strong federal power, inclusion of Indigenous peoples and minorities. The failures reflect deep divisions rather than unilateral betrayal. Since then, Quebec has operated with a "de facto autonomy" in several areas (immigration, language, etc.), but the constitutional file remains frozen. Governments like that of François Legault sometimes speak of conditional adherence, but there has been no concrete follow-up for now.
These events are part of the history of the "two solitudes" and largely explain contemporary Quebec nationalism. For more details, sources such as the Canadian Encyclopedia or books by historians (e.g. on Lévesque or Mulroney) are excellent.