The real reason, as always, is a mix of factors. It's the Chinese with their millions that they try to secure, the shear volume of immigrants and their quality, the fact that urban planners have focused more on compact accommodations like condos and townhomes rather than traditional detached homes, the green belt and of course the GREED of real estate firms which manipulate the market as they see fit to enrich themselves. 

Those who try to single out a "responsible" factor are doomed to fail.