1) the immigration rate is so high it isn't enough to keep up
2) the house values in areas that dont build in enough housing have skyrocketed, and smart homeowners in those areas have decided to sell and move to places with lower house prices and purchase more than one, which of course rockets the price even in the places that do build a lot. So even in provinces where lots of housing is built, immigration from other countries and other provinces blows up the housing prices anyways.
3) The government doesn't want to do anything about it because immigration is how they're keeping down wages, and since housing has become so much of the GDP they're terrified of what it would mean for the economy if there was a price crash anyways, so they don't look at policies that would fight the housing price explosion in fear of what the economy will look like if they do.