Why are there more deaths in Ontario and Quebec because of COVID 19 virus than all the rest of Canada?

Because 22 million out of the 37 million in the country live in those two provinces.

Duh.

Density of the population................................

The obvious answer is that more people live there.  And, they are not quite a strict as other places about following the guidelines set up by health departments.  Here, in Yukon we've been almost anal about the rules, even restricting movement within the Territory, let alone too it.  We had 11 early cases from people returning, but, nothing for quite a while now.  And, we still have stronger rules than down south.  Perhaps Ontario, and Quebec should take not of this.

Those 2 provinces have 2/3rds of the Canadian population.  So naturally they will probably have twice the covid deaths of the rest of Canada.  A better question would be why do they have 95% of the country's covid deaths

Simply because there are far more infections within the 2 provinces compared to all the other Canadian provinces and territories combined. As of May 26, there were a total of just over 90K cases in all of Canada.  50.6K were in Quebec and 27.5K where in Ontario.  The rest of Canada made up the remaining 12K

Because there are more lives in Ontario and Quebec than there are in all the rest of Canada.

As is true all over the world, crowded cities have more infections. Public transportation plays a role in that.