There are two reasons. First, history shows that homeowners will do anything to keep their home. They will miss payments on cars and everything else. That is why ‘power of sale’ listings are still low.
Secondly, investors have little reason to sell. If the property has positive cash flow, and most properties purchased a few years ago do; then why would they sell and pay capital gains tax? In the U.S., there are rollover provisions that if you sell and then buy in the same asset class, there is no tax until you dispose of the last property in that class. If we had that rule in Canada, you would see investors sell a lot more properties and that would increase the supply to the overall market.