You want an explanation? Here is one.
First in 1759 the French lost the war against the English in North America.[0] Canada, then known as 'New France,' came under English rule, and all the French elites that had the means to leave went back to France.[1]
The connection to France was severed. Thirty years later in France society would be upended by the revolution.
Now in the 18th century Paris there were two 'usages' or accents in current use. The 'bel usage' and the 'grand usage.'
The 'bel usage' was spoken by the monarchy down to the common people. The 'grand usage' was used for public speeches and by priests and politicians etc.
The bel usage was more familiar, more a relaxed pronunciation, with many elisions («leux valets», «sus la table») while the 'grand usage' was more 'pointu', that is to say more precisely pronounced, picking out each syllable more individually, with fewer elision and dipthongs.
The linguist Jean-Denis Gendron, in his book 'D’où vient l’accent des Québécois? Et celui des Parisiens ?'[2][3] argues that after the third estate won the revolution, they purposefully adopted the 'grand usage,' perhaps as a means to distinguish themselves from the old order.
Ironically, people in Québec now speak a derivative of the 'bel usage', which has evolved over time, but more slowly because Québec culture was very conservative even up to the 1950s.
It is said that when the king famously said: 'la loi c'est moi', it probably came out like: 'la loé, c'est moé' sounding like a Québecois.
[0] https://en.wikipedia.org/wiki/Battle_of_the_Plains_of_Abraha...
[1] This is the so called 'social decapitation' of New France. Only the priests and local school teachers were left to educate the people.
[2] https://www.pulaval.com/produit/d-ou-vient-l-accent-des-queb...
[3] https://www.immigrer.com/faq-dou-vient-laccent-quebecois/